The Telephone No. 394 was first issued by the BPO in 1937, principally for use on CBS and C.B. exchanges in rural areas [long lines], hence the provision of a battery-assisted microphone (local battery) and external connection of a magneto generator. This table model is Bakelite cased.

This telephone had a tray for the subscribers personal directory but as from June 1940 the tray was not fitted and instead the telephone was supplied with a Tray, Dummy, D62099 (a blank plate).  This was probably a war time cost saving, as in late 1947 the tray was re-introduced. 

During World War II, these instruments with their provision for up to three press buttons (keys), were also modified for use with 'Scrambler' Units, which were basically Frequency Inverters using thermionic valve.  Some were modified for Scrambler use and called the Telephone SA5031 and Telephone SA5063.

This phone was also used on long lines in C.B. and Automatic areas on extension plans.

Early units of the latter, referred to as 'Privacy Sets', were housed in heavy, bulky rectangular metal cases, but later versions were produced that became progressively lighter and more compact as smaller components, including more modern valves, became available. The most common 'Unit' was 'Frequency Changer No. 6', used extensively for war-time service (the post-war combination was the Telephone No. 710. and the very much lighter and compact transistorised version, 'Secrecy Unit No. 8').

Scrambler telephones always have green handsets, either moulded in green material or more commonly painted green. Similar-looking telephones can be found with red handsets but it is unclear what the other colour signified. Operation of the secrecy button simply disconnects the A and B lines  and the handset from the telephone, and reconnects both to the 'Privacy Unit' which now acts as the telephone, but now with frequency inversion facility.

Jim Foster states that if you were of sufficiently ‘high rank’ you had the exclusive use of a ‘Privacy Set’ and all you had to do was to press the button marked 'Secret', and lo! - secret it was. The 'Normal' key was not wired, and simply released the latch on the 'Secret' key, as did replacing the receiver on its rest. If, however, several users were obliged to share a common Privacy Set, then you had to press a button marked 'Engage for Secret' [or, on some versions, 'Hold Scrambler'], and a set of Doll’s Eye indicators showed if the Privacy Set was engaged. If it was 'free', then the 'Secret' or 'Scramble' button could be employed. Replacing the handset released any key that  had been depressed, but if you wished to revert to 'normal' speech in the middle of a ‘scrambled’ conversation, a third, central key, unwired, released the 'Secret' or 'Scramble' key. This button was however, marked on some ‘phones, 'Release', but where the telephones were connected to the line via a PBX, 'Recall', so that the user could re-seize the line. To add to the confusion, 'Exclusive' scrambler phones were often installed on multi-use positions, and vice-versa.

"Regarding scrambler telephone hand sets, as far as I have discovered the handset was supposed to be green with matching ear and mouth pieces. But I have in my possession handsets recovered from an airfield here in Cornwall that had ivory handsets painted green with ivory and black mouth and earpieces. Needless to say they were on scrambler telephones. I assume that painted versions overcame wartime and immediate post war shortages of green components. and nothing more sinister than that." Richard Killen.

Telephone Includes:-
1 x Bell No. 59A, Unmounted.
1 x each of Bell-gongs Nos. 2 & 2A Nickelled.
1 x Capacitor, M.C., No. 97.
1 x Tray-dummy, D 62099.
1 x Coil, Induction No. 21.
1 x Dial, Automatic, Dummy No.3, Black.
1 x Part No. 3/SCA/20, Black.
1 x Telephone No. 164 Black.

To be requisitioned separately as required:-
1 x Block Terminal No. 20/ ... , Blk.
1 x Cord, Instrument, No ...., 54' (see relative Plan No. diagram or where a cord longer than 54" is required 72", 120", 180", 300" etc.
1 x Cord, Instrument, No. 5/28E, 5".
1 x Dial, Automatic No. 12.
1 x Generator No. 4CN.
1 x Key No. 303A or 304A.
1 x Label No. 160.
1 x Label No. 252, Black.
1 x Parts No. 1/SPL/325

Note.-Where instrument coloured other than black is required requisition and assemble locally:-
1 x Dial, Automatic, Dummy No. 3 (Colour).
1 x Part No. 3/SCA/20 (Colour).
1 x Telephone No. 394 LB Black, minus Cord.
1 x Telephone No. 164 (Colour).
1 x Tray D 62076 (Colour).


Manufactured by: British Ericsson and Plessey, possibly others.

Colours: Black with green handset if used with Privacy Units.  Chinese red, Jade green and Ivory.

Users: Scrambler version used in government and military establishments. Examples can be seen in the Cabinet War Room museum in London, the Hack Green bunker museum ( and at the Yorkshire Air Museum, RAF Elvington, near York.

Mark 1 - has a Receiver, Inset No. 1L in the handset and switch bracket with plungers of the early type.
Mark 1A - has a Receiver, Inset No. 1L in the handset and switch bracket with plungers of an improved type.
Mark 2 - has a Receiver, Inset No. 2P in the handset and switch bracket with plungers of the early type.
Mark 2A - has a Receiver, Inset No. 2P in the handset and switch bracket with plungers of an improved design.

Authentic Telephone No. 394 with paper label of secrecy warning Telephone sold on eBay as 396LB. The arrangement may not be authentic

Supplied to the GPO by British Ericsson, their Model N8443.

Circuit diagram - N494.

Diagram for Button labels - N620.

Drawing No. 63022 - Telephone components parts.
Drawing No. 63023 - Chassis components.
Drawing No. 63250 - Chassis components (improved switch hook buttons).
Drawing No. 90319 - Telephone parts (Mk 3A).
Drawing No. 90373 - Chassis components (improved switch hook buttons).
Drawing No. SBA12 - 300 type base plate.
Drawing No. SCA17 - 300 type complete case.
Drwaing No. SCA20 - 300 type case.
Drawing No. SCA47 - 300 type complete case (improved switch hook buttons).

How to wire the telephone to work on the UK Plug and Socket system

Dismantling the Telephone No. 394

Dismantling the Handset

History, Technical and General Information on 300 type telephones

Collectors information

Adjustment of Magneto bells

Keys for use in 300 type wall and table telephones

Lamp Fittings

Labels that fit in the sliding tray

General fault finding on your phone

How to restore Bakelite

Comparison chart of GPO 300 type telephones

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Last revised: April 29, 2024