Interesting, does your cab have a plate on the front of the cabinet, to the left, right under the CP?
It'll say "NAMCO LIMITED" and has a few kanji characters in red.
If not, then I think at some point by 1980 or so they went away from the numbering system above and just went to these plates with sequential numbers, regardless of the model.
I might be mistaken, but IIRC, none of the cabs I have have the serial number like that on the rear of the cab!
Also, is there a Namco paper tag on the inside rear of the coin door? It'll have the production date/inspection date/ or some such stamped on it in Japanese format.