Well just to muddy the waters a little , the 40SR came out in '73 with rear drum induction in either pylon form for RC or rat race for CL. My engine in the photo above is the CL rat race engine. The crankcase has SR moulded underneath the exhaust port. In '75 OS made a front induction sport engine based on the same crankcase and called it FSR where the F presumably stands for Front induction (just a guess though) but the crankcase still had the SR under the exhaust. The RSR designation started when the rear induction models changed from drum valve to disc valve.