I thought the M designation was for the frame being modified? does the exhaust come under frame?
Yes. AMA Bonneville supplementals, page 29 - Production (P) Class frames
Production class motorcycles
shall not be modified from original equipment at time of
manufacture. e.g., frame, forks, gas and oil tanks, seat, front and rear
lighting, fenders, wheels, brakes, air intake box and (unmodified)
exhaust system.
Section 5.D., page 32 - Modified frames
EXHAUST AND MUFFLERS
Length of exhaust/muffler assembly shall not extend past the rear
edge of the rear tire. It must be sufficiently anchored.