The reason that serial numbers stamped into the BB of a 60s or 70s PX-10 make no overall sense is that they were not put there by the factory. The factory used a short metal strip with a number on it, riveted to the BB shell. You can clearly see the holes in Tim's photo ot the serial number. You can also see that the serial number was put in after the bike was painted.
Another way of dating a PX-10, which this seems to be, is to look at the shift-lever stop on the downtube. On models made before the long-point lug years (73-74?), the stop is a short piece of wire. After that, it is a slightly larger piece that looks a lot like the overlapping circles of a MasterCard logo.
I will also note here, having worked at Tow Path Cycles in Georgetown, which sold a lot of PX-10s in those years, that it is hard to find one with all of its original parts. The reason is that it was considered a nice frame with cheap parts, and was probably modified more than any other manufacturers model, often starting brand new.
I worked with a guy who bought one, tore it down to bare frame, had it repainted, then rebuilt it, keeping only the Stronglight 93 crankset and the leather saddle from the original equipment.
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