Hmmm... The front intake engines all had "F", and the rear intake had "R", as in ".61FSR" and ".61RSR". The front-intake, rear-exhust engine was a "VF", and the rear-intake, rear-exhaust engine was a "VR".

.60 FSR: http://www.osengines.com/history/ostimeline05.html
.60 RSR: http://www.osengines.com/history/ostimeline06.html

.61 VF and VR engines: http://www.osengines.com/history/ostimeline07.html

The airplane VR is here: http://www.osengines.com/history/ostimeline09.html it came out later than the marine versions.