OS have been doing that for a very long time and there doesn't appear to be any particular reason for whatever letters are used although obviously it'll mean something to the factory. The only one's of mine that seem to make a little sense are a couple of the old MaxIII-15's, where one has 62 A and the other has 63 H, and two Max-S 35's where one has 63 L and the other 66 L. These at least seem to show maybe the year they were made. With my engines, it seems that when they started with the Max-H series they switched to using just two letters. My Max-H35 has AV. However I have 3 OS Max 40 VF's and they're marked as AF, SY and VP. Go make sense of that .