O.S. made a .45 FSR ringed and a .45 FSR ABC. The "SR" meant it was Schneurle ported. The "F" meant it was front intake. O.S. made other engines that were "RSR" or rear intake engines. The rear exhaust versions were the "VF" and "VR" engines.

Usually, the case had a small "SR" just under the exhaust port. Because the center crankcase was identical in the FSR and RSR models, it would not have an "F" or "R" cast into the case.

There was no engine designated just ".45F". The FSR was sometimes written as "F-SR". The O.S. Max .45 FSR came out in 1978:
http://www.osengines.com/history/ostimeline07.html