A dynamo that's had a few adventures by the look of it!
It's possible it's a positive earth dynamo, ie the + and – are reversed, but assuming it's negative earth:
The Brown Wire is on the D Terminal, this is the main Plus winding.
Judging by it's length, and contrary to Andy's suggestion, I suspect the Green/Yellow wire once connected to the F terminal,. This is missing from the rectangular hole just below D.
F is the Field connection used to magnetise the dynamo; it's normally connected to a Regulator which alters the field current as necessary to maintain a steady 12V out whatever rpm the engine is turning.
I've never tried running a dynamo as a motor but guess the F wire and D terminal should both be connected to +12V from the battery. The dynamo might motor without energising the field, but at low power.
Three wires is a bit odd because dynamos are normally earthed by bolting the metal case to the chassis. I guess someone has bodged this example in the past and the black wire is earth, ie to be connected to -12V from the battery.
Do you have a multimeter? A cheap one would soon remove all doubt.
Dave