In the process of collecting Cycles Gitane serial number data, I discovered that there was another person attempting to do what I was do what I was doing.

ZZ on gogovelos a bicycle forum with primarily French contributors.

Like me, ZZ had determined that where there was a uniform middle row that this was a frame model designation. ZZ had also determined that there was a consistent four digit group of numbers.

ZZ had made two determinations that I had not.

First was that the trailing character after the three or five digit frame designation was for the frame size. Wonderful. That makes sense.

Second, ZZ had determined that the last digit in the four digits was for the year.
Wow. That is a big step in looking for patterns.

With these Two things, ZZ had also determined that the three digit frame designations began in 1972
and the five digit designations began in 1974.

I applied this new insight to the serial numbers on the dropouts. I now knew to order them by year.

From the data that I had to that point, it appear that the first three digits were for day of the year. This would be the same as Schwinn bicycle date codes on the head badge.

I posted this on the Gogo forum and ZZ was unimpressed.

I went back to collecting more data.

After collecting more serial numbers I began a new analysis of the four digit group.

The range of numbers in the first position was 1 to 7.
The range of numbers in the second position was 0 to 5.
The range of numbers in the third position was 0 to 9.

Well there it is. First digit is day of the week. Middle to digits are week of the year. Last digit is year.

I checked this with the four digits from the bottom bracket and it held true.

I went to post this on Gogo in October 2021, only to find that ZZ had come to this same determination in September.