The AMA rulebook takes the worst of the SCTA wording. Good luck trying to figure out ANY class combination. This must be the worst rulebook in the world. Oh, wait . . . That would be the FIM rulebook. No wait . . . Is it the SCTA rule book? The all suck when it comes to clarity of rules.
Unlimited is just like it sounds. Open to any unconventional means (Wankel for instance) of producing power. Stainless is correct, the old school method of advancing in class displacement to even the playing field for turbochargers is what it refers to. Long dead.
I don't think anybody that actually runs has anything to do with the rules.
"Chapter 5 MODIFIED (M) CLASS - 5. J. OPEN CLASS*
Open class motorcycles do not permit streamlining."
And then it goes on to tell you what streamlining is permitted.