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Not exactly. FMM means that is an air-cooled, vertical, '250'cc class motor. The 1 is for single cylinder, and the 2nd number, like 63 or 56, is bore. Zongshen made several 167FML motors, a pushrod CG clone, an OHC CB clone, and an OHC Yamaha clone, just as an example.
There isn't anything denoting if it's pushrod or OHC, just pictures. Valve actuation never figured into their coding of the engine name. To add confusion to that, early on (I've got an '05 that corresponds to this), sometimes the name didn't actually reflect what was actually in the engine, i.e. I have one labeled 167FML that has a 63.5mm bore. Earlier Chinese stuff isn't too consistent.
I'm pretty sure that they've used this system since Chinese motorcycles were imported en masse to the US.
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