There are two styles of 3 brush dynamo, early fixed 3rd brush and the later adjustable 3rd brush type. You can adjust output with the moveable 3rd brush style.

Check out https://www.bolsover.com/lucas/index.htm for good information on testing generators.

You can opt to discard the third brush and run the dynamo with a modern electronic field control.
Look in https://www.norbsa02.freeuk.com/goffyelectrex.htm for more details.

I do not know of your model, but guess that it has the 3rd brush earthed. It will only give an output if rotated in the designed direction.
Before stripping it, remove the 3rd brush and see if it produces voltage output. Without the 3rd brush it should give an output when rotated in either direction.
Do not have any lights connected, or you may blow them. The dynamo will be unregulated and it should produce about 18 volts without the third brush.