Good news and bad news.
The good news is that I have indeed found an identical microphone from the handset of a dead
Duet 200 telephone - and the problem of the buzz is solved.
The bad news is that I now have a new problem to track down. I must have disturbed something else, as the duck has lost its "quack". There is neither sound nor flashing eyes on an incoming call. These were both working before.
Edit: ignore the above sentence. What I had disturbed was the "ringer on/off" switch. Unfortunately, in the "on" position I now get a continuous "quack, quack, quack ..." while the 'phone is on-hook - the equivalent of the continous ringing that can occur if A and B legs are transposed on a three-wire connection. Only this uses a two-wire connection.