Pictures if possible; I've seen faked chrome stays. I would do the following:
1) Check for "simplex" engraved on the rear dropout, over the derailleur.
2) Check for Reynolds 531 stickers on the frame and fork. These are often partial.
3) Look for the word "inoxyadable" on a sticker on the seat tube; these wear better than the 531 stickers.
4) Make sure it has an integral derailleur hanger
5) It should be either blue or white in color if the original paint is still on it. I've seen black, but in a very limited edition.
6) Read the following links:

http://www.classicrendezvous.com/Fra...ugeot_home.htm

http://www.classicrendezvous.com/Fra...10_history.htm

The serial number seems a bit off; sixties Peugeots have six digits; early to mid seventies seven.

If it is a PX frame, and it's your size, straight, true, no dents or dings, and the price is reasonable (the frames go for between 50-100$ bucks on ebay, sometimes more depending on vintage) I'd pick it up. They make great fixed gears. Check the headset and BB cups; if they're original, they're stronglights, which are built like tanks and pretty hard to destroy. If they're in good working condition, it saves you the difficulties of finding French replacements.