yeah if you look in the parts books or on alpha-sports you can see that there was an optional 'sound kit' i bliev it was called. This is nit for road registering a RM, but i reckon it was for in some areas in US, probably california to make the bike quiter and theres probably a spark arrestor in the muffler too. Was probably for desert racing or strict riding areas. Also in the kit was a different air box lid and rubbers for the cylinder fins/head and a couple other little things.
Remember more noise doesnt equal more power. That muffler probably doesnt rob as much power as you think it would.
