[BakeliteBear]

Thanks for the encouragement Nick!

In this instance it was a 332.

I agree that the ‘trial and error’ approach is probably the best practical solution, but I would like to understand the theory behind it if anyone can help. Surely all models of a paricular telephone should behave in the same way?

Incidentally a missing strap from the dial terminals on the chassis between 2 and 3 will cause horrendous noise - so its one to look out for!