I think that the body of the dynamo should be stamped with the model of the dynamo. It is most likely to be a C35. The C35A has brushes with no wires attached, the C35M has brushes with wires. You can see pictures of BG154b & BG293b here.

Melvin's Dynamo Photo uploaded on his behalf:

Looks to be a C35M - the letters beyond more than likely BN30 - the 7 after the arrow likely to signify a 1937 build date - part number's probably 220299.



Jeff.

Location: Almost but not quite, the far North East of England