There was another document that listed the codes used for all 'engineering' premises such as exchanges, repeater stations, radio stations, TV transmitting stations.
The document was known as E-in-C 1141 and still exists today with the 1141 as part of its title.
My copy quite early as it mentions TV companies like TWW (Television Wales and West) that was replaced by Harlech TV (later becoming HTV) in 1967. Moel-y-Parc TV transmitter which opened in 1964 and Cairngorm exchange which opened in Jan 1964 aren't mentioned either so puts it before that date.
The 'engineering' codes are sometimes the same as the operator codes but different at other times.
For instance I have a small 20 line CBS3 wall mounted public exchange switchboard and the lines to the 'parent manual-board are labelled '
FGS'. There is no engineering code '
FGS' - The only exchange I could find was '
Fort Au
Gustu
s' which had one of the 470 CBS3 exchanges that the GPO had connected to it by the name of 'Glenmoriston' exchange that used to be in the
then village post office at
Invermoriston near Loch Ness. After a more research I was able to discover evidence to back my theory up. Oddly the numbering scheme on the exchange was 201 through to 220 but with 221 instead of 211 ! I discovered that Glenmoriston was renumbered in 1950 from 1 - 20 to the above. Then I managed to track down someone who use to work the switchboard before WW2! And they conformed the FGS without my even mentioning it! Surprising what you can find out! The manual exchange was replace by a
small automatic UAX12 exchange half a mile along the toad towards Inverness