A 1051
Issue 1, 16/6/56


1. Scope of Instruction
This Instruction describes keys for use within 300-type table and wall telephones and the key plungers and labels used with them.

The information is given in the order indicated below:

  • General
  • “Keys Nos. 302A and B”
  • “Keys No. 303A”
  • “Keys Nos. 304A and B”
  • “Keys No. 303B”
  • Setting of latch-bracket control plate on “Keys Nos. 303 and 304”
  • Gravity-switch release of normal "Keys Nos. 303 and 304”
  • Plungers for use with “Keys Nos. 302, 303 and 304”
  • Labelling of keys
  • Label fixing

  • Reversed labels for wall telephones

3. General
Keys Nos. 302.., 303... and 304... are fitted within 300-type telephones to perform auxiliary functions for extension plan working, for shared-service signalling, and for extension bell control, etc.  The telephones in which they are fitted are described in A 1010 (table telephones) and A 1050 (wall telephones).  Diagrams of the connections of the keys, and other supplemen­tary information, are given in Diagram N 4301.

4. “Keys Nos. 302A and B”
These are single keys and each pattern is similar in construction.

“Key No. 302A“ has one make-before-break contact, and “Key No. 302B” has one break-before-make contact.  The keys may have a locking or a non-locking action, depending upon the types of plungers used with them.  The different types of plungers are described in par. 10.  The keys are fixed within telephones by screwing the two trapped screws (parts of the keys) into threaded pillars on the table telephone chassis or wall telephone base.  A “Key No. 302“ is illustrated to the right.  The keys are connected to the telephone terminals, as required, by means of short lengths of wire soldered to the tags on the spring-set.

The keys are now stocked complete with these wire ends soldered on, under the titles “Key No. 302A-l” and “Key No. 302B-l“; previously “Keys Nos. 302A and B” were used with wire ends which were provided locally.

5. “Key No. 303A”
This is a multiple key having three separate spring-sets which are individually controlled by the operation of three plungers fitted in the telephone case.  The A and C spring-sets each have four make-before-break contacts and the B spring-set has one make-before-break contact.  The A and C spring-sets can be arranged to be non-locking, or to be locking with release controlled by the other plungers, as determined by the setting of the latch-bracket control plate described in par. 8.  Locked spring-sets may also be released by the gravity switch of the telephone in which the key is fitted, as described in par. 9.  The key is fixed within the telephones by screwing the four trapped screws (parts of the key) into threaded pillars on the telephone chassis or base.

Key 303A - rear view Key 303A - front view

The springs of the spring-sets are wired to screw terminals on a terminal block forming part of the key.  Connections between these terminals and the terminals, of the telephone are made with insulated wire, or cords are directly connected to the key terminals, to meet circuit requirements.  A “Key No. 303A” is illustrated above and is shown, in the picture to the right, fitted to the chassis of a table telephone.

6.    “Keys Nos. 304A and B”
These keys are identical in construction and method of use with the “Key No. 303A” (described in par. 5), except that they are fitted with a smaller number of springs per spring-set and a terminal block with a correspondingly smaller number of terminals.  The A spring-set of the “Key No. 304A” has two make-before-break contacts, and the B and C spring-sets each have one make-before-break contact. “Key No. 304B” has similar spring-sets but the contacts are of the break-before-make type.  A ”Key No. 304A“ is illustrated in Figs. 6 and 7.

Fig 6 Fig 7

7. “Key No. 303B”
This key has been specially designed for use in telephones associated with privacy equipment.  It is generally similar to the “Key No. 303A” (described in par. 5) but has only two spring-sets, the B spring-set being omitted to make room for a fifth make-before-break contact on spring-set A.  Spring-set C has four make-before-break contacts.

8. Setting of the latch bracket control plate on “Keys Nos. 303 and 304”
The locking and release actions of spring-sets A and C on “Keys Nos. 303 and 304” depend upon the setting of the control plate, which is fitted to the latch bracket, as shown in Fig. 8.  The control plate may be fitted to the latch bracket four different ways round and, by using alternative holes for the screws which hold it to the latch bracket, it can take up three different positions upon the bracket. The alternative ways of fixing the control plate enable seven different interlocking conditions to be set-up.  Diagram. N 4301 lists the different conditions and shows the position of the control plate for each.

Fig 8 Fig 9

9. Gravity-switch release of locked spring-sets on “Keys Nos. 303 and 304”
 By the use of an extension arm, which is supplied fitted to the keys in a storage position (see picture above) locked spring-sets of  “Keys Nos. 303 and 304” may be released by replacing the handset on the telephone.  The extension arm is used in different ways on table and wall telephones when this facility is provided:

(a) On table telephones, the extension arm is transferred from the storage position on the key to the gravity-switch bracket of the telephone.  When the telephone handset is replaced, the extension arm strikes a projection on the latch bracket of the key and causes it to release any locked plungers.  When a key is recovered from a table telephone, the extension arm should be refixed in the storage position on the key.

(b) On wall telephones, the extension arm is fitted to the latch bracket of the key, as shown on the right.  The screws which hold the control plate to the latch bracket should be used for this and, at the same time, the control plate should be positioned to give the  locking facility required.  The screws by which the extension arm was fixed in the Storage position should be replaced in the storage holes. When the telephone handset is replaced, the extension arm is struck by the gravity-switch bracket and any locked plungers released.  Early supplies of keys were fitted with an extension arm with a shape unsuitable for providing gravity-switch release on wall telephones.  The new pattern extension arm is available as a Rate Book part (Part No. 2/SPL/488) and should be requisi­tioned if it is necessary to give the facility with an old-pattern key.

10. Plungers for use with “Keys Nos. 302, 303 and 304”
A range of four plungers is available for use with these keys, the particular plunger to be used depending on the type of telephone in which the key is fitted, and the facility required.  The types of plunger are as follows:-

(a) Part No. 1 /SPL/32. The plunger, illustrated below, is for use with "Keys, Nos. 302, 303 and 304” in table telephones.  When “Keys No. 302” are used with  this plunger, they are non-locking


(b) Part No. 1/DPL/361. This plunger (illustrated in Fig. 12) is for use with “Keys Nos. 302, 303 and 304” in wall telephones.  When “Keys No. 302“ are used with this plunger, they are non-locking.  The plunger differs from “Part No. 1/SPL/325“ in that it is longer, and it includes a light Spring which ensures that, when a locking key is pressed, the plunger remains depressed to remind the user that the key is operated.  In table telephones, gravity makes the use of a spring unnecessary.

(c) Part No. 1/DPL/358. This plunger (illustrated in Fig. 14) is for use with “Keys No. 302“ in table telephones.  When used with this plunger, a “Key No. 302” can be locked; this is done by rotating the plunger 90° clockwise, after pressing it.  A metal plate which forms part of the plunger then moves so that it engages with a thickening of the case moulding around the plunger hole, preventing the plunger being pushed out by the spring of the key.  The key is unlocked by rotating the plunger anti-clockwise.

(d) Part No. 1/DPL/365. This plunger (illustrated in Fig. 13) is for use with “Keys No. 302“ in wall telephones when the locking facility is required.  It is similar to “Part No. 1 /DPL/358“, except that it has an extension to bring it to the correct length for a wall telephone, and it operates in the same way.

11. Labelling of keys
The plungers are labelled by attaching to the telephone cases either “Labels No. 252“or “Labels No. 352“. “Labels No. 252" are anodized-aluminium labels and are stocked with a variety of lettering; the different types available are illustrated in Dgm. N 620 and are listed in the Rate Book.  “Label No. 352“ is a plain plastic label for use where a suitable “Label No. 252“ with the required lettering is not listed.  “Labels No. 352" will be individually engraved by the Factories Dept. to the user’s requirements, which should be stated on an engraving schedule (A 1098) accompanying the requisition.

12. Label fixing
The labels are fixed to the telephone case by two screws.  In one method, the screws have slotted heads and are screwed into metal bushes pressed from inside into holes in the case moulding.  The alternative method uses screws with unslotted heads.  These are passed through unthreaded holes in the case and are held by slotted nuts on the inside.  The necessary screws and nuts for one method or the other are included as parts of the cases of telephones to which keys may be fitted.  The first method has been used hitherto for table telephones; the later method is used on wall telephones and will be extended to table telephones in the future.

13. “Reversed“ labels for wall telephones
Because of the way a “Key No 303 or 304” is mounted in a wall telephone, the order of the plungers from left to right, as viewed by the user, is reversed compared to that in a table telephone.  If the connections to the key in a wall telephone are made identical with those in a table telephone it may, therefore, be necessary to fit a “reversed“ label.

By an interchange of the connexions between the A and C spring-sets of the key, it will be possible in most cases to use the normal “Label No. 252“.  Where this interchange is not possible, the key Spring-sets A and C should he wired in accordance with the N diagram for the table telephone concerned and a “Label No. 352“, engraved with reverse labelling in the A and C positions to that of the normal "Label No. 252“ should be ordered from the Factories Dept.


BACK Home page BT/GPO Telephones Search the Site Glossary of Telecom Terminology Quick Find All Telephone Systems

Last revised: June 15, 2015