[TonyDuell]

To answer some of your comments :

Yes, Relay A is the relay in the plinth. I think on a Mk3 Planset it's one of those little 'continental cradle relays' as RS used to call them. The convention on GPO diagrams is to give the coil a letter (here 'A') and the contacts operated by it the same letter followed by a numner so 'A2' is a contact operated by the relay coil A

The LS reed relay is a long cylindrical thing in the plinth. But that all seems to be working correctly, the relay 'A' is operating as I would expect. And the fact that the buzzer operates when you press the armature of the relay in the ringing converter confirms this.

And yes, the problem is that the relay in the ringing converter is not operating on incoming calls with Ext-to-Exch pressed. The wiring of this relay coil to the plinth seems correct.

Can you confirm that the extensions ring on an incoming call with Ext-to-Exch pressed? If they don't then there is possibly a problem there (the extension bells are in parallel, that combination is in series with the relay coil essentially).

If the extensions do ring on an incoming call, try (temporarily) disconnecting one of the relay coil wires at the ringing converter terminals. Does this stop the extensions from ringing? It should do as it would open the circuit to their bells. But if it doesn't there's a short or misconnection somewhere.

Another silly thought... It's not the diode in parallel with the relay coil in the ringing converter its it? If that diode is open-circuit I have an idea the relay will not respond to the AC ringing signal. It will burn out if you test the relay on DC of the wrong polarity.