Honda 120 Identification

Can someone help me identify Thailand vs Japanese 120's and for that matter 160's also? I've been aware of a head casting difference on the 120's but the tech manual doesn't seem to address the difference other than excluding Thailand motors from USAC competition. The Honda web site indicates a stamped ENGINE TYPE CODE, but I cannot locate a guide to decode them. Curiously both USAC and QMA tech manuals list the same engine series identification so they don't seem to address it either.

Maybe I've overlooked something.

Thank You