I've only seen AX, FX, and SX on the OS engines...

The AX and FX seem to be their high quality "ABL" (Advanced Bi-Metallic Liner) engines with ball bearings. The SX engines appear to be bored & stroked versions that have higher displacement in the same size engine (.50 SX is same size as a .46 AX/FX, .32 SX is same size as .25 FX), and the .50 SX is actually a ringed engine, instead of an ABL engine like the .46 AX/FX. OS also makes an LA series, which I believe are lapped engines (no liner) with bushings (instead of ball bearings).