A factory at Romilly-sur-Seine was purchased by Cycles Peugeot in 1972. It had been used to manufacture refrigerators and therefore need some time to convert to bicycle production.

Early in the production history of this factory, bicycle frames had a production code stamped into the bottom bracket shell. All but one of the production codes that I have seen are four characters in length.

The production code started with the letters “RY”. I presume that RY stands for the factory at Romilly.

The third character is always a letter in the range from A to M, but I have only seen ten of the letters in this range. The fourth character is always a number 4, 5 or 6.

I presume that the last two characters are a date code. The letter is for a month of the year and the number is for 1974, 1975, or 1976.

This date code, which is on some Peugeot frames, is not a serial number or a frame number. Frames with a date code also have a serial number stamped into the dropout or an an aluminum label riveted to the bottom bracket.

This date code is not found on all Peugeot models. This date code is seen most often on PX50 and UO8 models. It has also seen on models reported as AE8, AO8, and UO10. I have never seen this date code on PX/PR/PA10 models.

In 1976 Cycles Peugeot changes its frame number/serial number/date code system. The new system included serial numbers and frame numbers with date of manufacture information encoded and a code for the factory.

The new system encompassed bicycles manufactured at both the Beaulieu and Romilly factories. This new system started several weeks or months into 1976. There may have been a period of overlap with the previous systems.

https://palaeobicycleology.ca/2021/11/cycles-peugeot-date-codes-an-introduction