This is not gospel. It's just based on what information David Goerndt and I gathered early on when he was building the original PX-10 database pages. He later was able to score a LOT more information.
The ones from 1979 on, stamped into the BB shell, do have a date code - the first numeral is the last digit of the year produced.
The most common ones were from 1970 to 1978 and had the riveted sn plates. When we were gathering data on PX-10s, I could find no correlation in the sampled Peugeots between the aluminum plate numbers and other dating information. Lemme stress that the sample was not as large as could be gathered today. Bikes clearly from the '75-78 era frequently had lower numbers than obviously earlier bikes. I think the aluminum plate numbers are like the serial numbers on Fender guitars from the old days - they had a bunch of plates stamped and they dumped them into bins, and people fished out plates as they went and slapped them on the finished product with no cares about what order they went on in.
Seriously, I think the Peugeots from 70-78 are best identified through paint, decals, and date codes on original parts.
Pre-1970 with numbers stamped directly into the BB shell? I dunno. I had a theory for a while there that the first digit of the sn was again the last digit of the year produced.
Feel free to correct me, just be gentle with me!
Russ Fitzgerald
Greenwood, SC