Thomas Crame wrote:

Circuit prints don't show the area code, just the signal number. G5
signal's circuitry for instance would have the circuit title 5G, and
its signal relay would be 5GR. Saying that, as every sheet would have
"INTERLOCKING MACHINE ROOM 'G' - GOLDERS GREEN"
at the top the process to change every master by hand could get a bit
labourious.

Wow, I never knew that the signal codes were that extensive!