This is a replacement muffler for the OS 61 FSR. This muffler has the bolt thru mounting. The bolts go through the muffler and thread into the engine case. Screw hole diameter accommodates 4mm screws (.172" dia.) and spaced 1.339" (34mm). Originally numbered 744 by OS but check yours carefully. Made in USA by MECOA in Irwindale, California.

May also fit with slight modification:
O.S. .50-.61 FSR
ASP .61
Enya .61X
MVVS .61
Webra .61-.70 F

Includes two 4mm socket head mounting screws.