Some progress.
The dynamo is back together and, with nothing else connected, spun over with rope gives nearly 3 volts which is very positive. The biggest job was cleaning decades of goo from inside the dynamo casing. In places it was over 1/2" thick and completely blocked the dynamo's drain. I suspect that is an indicator of the state of the engine

Incidentally I was wrong to think it compound wound as its plainly shunt. GEC also marked each lead with their usual letters: A, AA; Z, ZZ and Y, YY.
cheers
Roland