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.