I read that it would be 6 or 7 digit serial number, mine has 5.

My understanding is that 1971 was the first year for this style of Peugeot downtube decal. Also, your Normandy Lux Competition hubs have the earlier gold label. The Nervex Pro lugs as on your PX-10 went away in 1972 but reappeared in in 1973 and 1974. However, 1973 and 1974 models had the Reynolds sticker located on the downtube rather than on seat tube like yours. For that reason, I'd say that you have a 1971 model.

On my 1973 PX-10, there is a date code on the handlebars of 1_73. If yours is dated, it may indicate a date of 1971 or late 1970.

Perhaps your frame was fabricated in the late 60's and not built for shipment and sale until a couple of years afterwards.

BTW, congrats on this terrific find.

There will be a date code on the rd cage as well

Yes, sometimes. Unfortunately, mine had no date. The only date I have is on the handlebars.

I have a 71 and a 72. Decals, lugs, angles, rake all say 71 to me. Serial numbers are pretty unreliable. That's going to clean up nicely.

Here is the serial # on my ‘71.
Frame tube decal on seattube.
Mine does not have a seamed BB.

71 is a good guess. Plain black plus and it's a 72 but that is all I know. (Yours has fancy lugs)

The first year for that downtube decal was 1971. Then, in 1972, they changed from the fancy Nervex Professional lugs to the simpler Nervex Dubois lugs. In 1973, they moved back to the fancy lugs, but this time the 531 decal was moved to the down tube from the seat tube. So, having the fancy lugs with that downtube decal, and the 531 decal on the seat tube, this is a 1971 PX-10.

When did they have the serial on a separate piece of metal riveted to the BB? This BB has the holes in the right place for the rivets, so maybe it fell off?

Some municipalities back then would stamp a serial in a bike that didn't have one, a requirement to get the bike license sticker. OP's serial sure looks like it was stamped later; they even had to put some touch-up paint over it, almost certainly because the frame would have rusted from being stamped after being painted originally.

Top is my 1971, below is my 1972. I always imagined there was a box of them, and after a frame was painted a random one was grabbed and riveted on.

Your serial number plates make sense -- 7 digits, leading 1 and 2 for 1971 and 1972. My 1970 UO-8 frame fits that same pattern, with a leading 0.
I suspect the other samples in this thread had label plate-ectomies. This was common practice: either remove or keep the original plate, and then stamp or etch some form of identifying number into the bottom bracket itself. Bicycle theft was particularly rampant in the early 1970s.

Am I mistaken? - or is the BB in post #1 the only seamed example?

I wondered the same thing

I never heard that about Peugeot serials. Must find more...
I was wondering after I took the pics of mine. Seems odd that a PX10 would have a seamed BB, n'est-ce pas?

Probably lots of them (or all of them) are seamed there, but the seam is ground flat on the others. Probably done at the BB shell factory, not by Peugeot.

There are two kinds of shells that don't have a weld, namely (1) cast and (2) bulge-formed. I don't think Pug used either of those types, I think they're more likely all welded.

Leaving the weld visible probably saved them like 2 centimes, so it's no wonder most of them have it ground flat, a much better look.

So mine was built on a Friday, and they just got lazy.