I'm trying to date my PX-10 and the serial number doesn't seem to follow the pattern described here:
Apparently "there are six digits used on bikes from the 50's and 60's and seven digits used on bikes from the 70's." However mine has only 5 digits (726**). It's stamped on the bottom bracket shell and it definitely does not look as if a number was scratched out or not embossed deeply enough. I'm pretty sure there was never a plate with a serial number on the bike; in all the pictures I've seen of the plates they are riveted on in two places. There is only one hole in the bottom bracket on this bike, and I'm pretty sure that's the drain hole (sorry, don't know the technical term).
My father bought the bike in Cambridge, Massachusetts in 1970 (for something like $250). The bike has the black and white paint job, fancy Nervex lugs and fork crown, and chrome tips on the fork and stays. I've used this bike since I was in high school in the late 80's and would never sell it. I'm just trying to find out more about it.
I'd be happy to provide more info if it might help someone make a better estimation.
PS Is there any compelling reason NOT to list the complete serial number? I don't think it's a problem for a 36 year old bike but I'm a bit paranoid.