JACK No's 620A, 621A, 622A & 623A

These Jacks were the original Line Jack Units. They were all surface mounted and some had covers that stopped removal of the inserted plug.

The jack entry was also on the side which made inserting a plug a little difficult at times.

The original PST sockets were:-
Jacks No. 620A - master, surface, with captive lid.
Jacks No. 621A - master, surface, with non-captive lid.
Jacks No. 622A - secondary, surface, with captive lid.
Jacks No. 623A - secondary, surface, with non-captive lid.

Circuit diagram of Jacks (Line Jack Units (LJU's) are wired the same except secondary has no components)
Master type is shown to the left and Secondary type to the right

These jacks (shown above) were coloured light grey and had screw terminations. The socket outlet was on the side and not the top. The captive lid variant stopped removal of the telephone plug, which could only be released by removing the lid with the aid of a screwdriver. See picture below.