I have drilled out the mouthpiece rivet and can report:
* blu/whi are used for transmitter
* transmitter is a Receiver 4T in black rubber mouldings. Not enough space for the mouldings and a Transmitter 16, and presumably they wanted to reduce handling and wind noise. There is a purple plastic sheet to spit-proof the transmitter
* org/blk are joined together and sleeved in the transmitter compartment, presumably to provide a cut cord detection circuit
* red/grn go up the handset to the receiver. I haven't drilled open the receiver as the damage would be noticeable there, but it clicks nicely on a PP3 battery and I assume it's also a 4T.
*there appears no way of undoing the armoured cord entry and the wires are very reluctant to pull through any longer.
I was a bit surprised the mouthpiece was only riveted, and not glued shut as well, I don't think it would be possible to re-rivet it closed after drilling out, so the entire handset must have been treated as non-repairable.
Having now verified the handsets are all intact I can get to work on the circuit. I'm hoping that simply removing the resistor providing battery voltage to the carbon transmitter will do for starters ...
The circuit I'm building is here
https://www.vintage-radio.net/forum/...8&postcount=20
"741-field-telephone-system-intercom-PW1980.pdf"