Rear swing arm, thats it. parently its differant width and shock pos by a few mm,
from what i can tell, got both here, but only look differant if you squint.
I have put it down to just one of those things, if you can get it changed, i got a few 75 here i could flood the market with and get a few more folks on the track racing. ultimately its not a Rm, but then again its a 75 model and them the rules, ( AKA the MX250B etc)