I'm a dummy. I forgot to mention the weird serial numbers on a small number of PX-10s from the 1970-78 period. Worse still, I owned two of them!
The pre 1970 ones had serial numbers stamped in line with the BB spindle axis, right? Those numbers were nicely stamped, if memory serves. The post 1978 serial numbers were stamped perpendicular to the spindle axis; 1979 on, I think they had an alphabetic prefix. Again, these were nicely stamped.
The overwhelming majority of Peugeots from '70 to '78 that I've encountered had the infamous aluminum plates discussed earlier. But I had two bikes that broke that rule. One, a PX-10E (black Nervex Pro lugs) that I bought at the very first Cirque, appeared to be from 1973, judging by some markings on the probably original Simplex rear derailleur. It was no. 882 33, stamped forward of the seam, on a BB shell with a drain hole and two holes similar to those used for mounting the riveted plate. The numbers looked to be hand-stamped.
Strangely enough, I also had what I believe was a '74 PX-10LE (smooth lugs, gold lug lining) with what appeared to be hand-stamped serial numbers. SN 44931 4, stamped forward of the seam, with NO drain hole or rivet holes. The stamping looked crude to my eye.
Both of these bikes were 60 cm, c-t, and had surprisingly steep angles - they weren't the usual 72 parallel frames. I remember David G. mentioning that someone who communicated with him about a similar frame. There were a couple of theories - either these frames had been made for a team, or they were warranty replacement frames.
Russ Fitzgerald
Greenwood, SC