To answer your original question on model letters, the first modern, schnuerle ported, ball bearing OS engines were the FSR series in the 1970s.
Following this series were the long stroke SF engines.
The FX engines were the next major series.
These are being slowly replaced by the newest AX engines.

The FP engines were OSs early plain bearing, lower cost engines.
These were replaced by the LA series engines.

In among these main series are several specialty engines (not counting the heli engines). The ducted fan engines used a V designation like VF or VR, as I recall for front or rear intake.
The SX engines like the .32SX and .50SX are low production engines which are supposed to be of higher quality with a resulting higher price.