RE: OS 55AX and pitts muffler
We're the O.S. support center, so WE cannot obtain non-O.S. parts for you. You will have to order from someone who sells Bisson mufflers.
Regarding the one for the .55, It may have a slightly-higher volume or flow-through than the one for the .46, or it may just be rebadged for those who MUST have one specific to their engine. Please contact Bisson for details.