just guessing, I would say its for the later models with the 3 spoke wheels. If you make the 17" conversion, you already have a sprocket. (it fits the 907 with the 17" wheels, and that's probably how he got it when a parts counterperson didn't know the difference between the 906 and the 907)
Cheers,
Duc in the Bay
1990 750 Sport x2-Rosso Blanko (900ss copy) & Nuovo Nudo (Scrambler project)
1991 907 -mostly stock
2002 ST4s - Lots of mods.