The "slotted" ones you see in the parts fiche are the -323- series advancers (most made by TEC). They are a larger round-ish baseplate than these, too. The -323- have a different static timing and advance angle (and rate) from the early engines, being made to operate with late-opening intake cams (0 BTC).
The "300" definitely puts them into the pre-1975 engine. The advancers were made by 3 companies at different times, mixed back and forth between New Factory and Old Factory, with the bulk of pre-1972 versions made by Hitachi. This logo looks like 2 concentric circles with a sort of "H" in their midst. The TEC is distinct, and the ND shows up, usually in K1 bikes, pretty often.
The curious part here (to me) is the slots behind the springs. This makes it look like it started out as a 450 twin advancer plate, which may be how it came to be? Those were largely TEC-made. The sandcasts sported many parts that came from the Big Twin. If the bolster in the center were then changed to fit the 750 shaft, that might be their origin?
Cool stuff!
