Yes indeed, this is the FSR from the days when OS made real quality. One for the best ever made by OS, my opinion only!

Do not try to read to much into the OS nomenclature, it will give you a clue but not all letters mean something! The FSR was available ringed or abc, and the rear exhaust model was called VF or VR depending on intake location. There were also RSR models available in some sizes.

But to your question, RJL/MECOA has "Replacement muffler for OS FSR .61" for $21.99. You will not find it on the net pages but it is shown in their catalogue.