I honestly don't know, but I think I've seen more S models than Ms, although hard to say if those S's were real ones or kitted M's. That said, the M and S are very similar. I think they are the same frame and running gear and bodywork. The differences are what? Cylinder porting, big carb, pipe? The M was quoted at 23 HP and the S at 26, so there wasn't a lot in it really.