For lack of a better term I am calling the numbers stamped into Gitane frame between the years 1969 and 1982 serial numbers.
There may be a better term.

Gitane serial numbers have two components, a four digit date code and a uniqueness number of a varying number of digits. I call the one group of digits a uniqueness number because I assume that it makes the serial number unique for a given date code. I do not know if the uniqueness numbers are sequential as I do not have enough data.

Gitane date codes I assume give the date of frame manufacture.

location of the serial number:

There are two locations: the dropout on the non-drive side or the bottom bracket.

Format of the date code:

Four digits.

Format of the uniqueness number:

Varies. Generally one, two , or three digits followed by spaces and another digit.

On pressed dropouts the format is usually a single digit followed by four digits.

Stamping of the date code on dropout:

The data code digits are stamped uniformly and spaced across the bottom tine of the dropout.
Note that the uniqueness number is also stamped on the dropout.

Stamping of the date code on the bottom bracket :

(ignore the frame model number if present on the BB)

There are three different arrangements:

1. Initially, the date code and uniqueness number were stamped in a single row.
   The first four characters are the date code. The following characters are the uniqueness number.
2. Later, the date code and uniqueness number are on separate rows. The top row is the date code.
   The second roe is the uniqueness number.
3. In the first quarter of 1980 the format switches to the uniqueness number is the top row and the date code is the second row.
   I have seen two examples of this in early 1979.

Interpretation of the date code:

digit 1: day of the week.

Digits 2 – 3: week of the year.

digit 4: year of one of the decades 1960, 1970, or 1980.

In the four example image above here is how the date codes decipher:

4281: day 4 of week 28 of year 1971. Manufactured in July of 1971. note that the “8” and “1” are upside down.

1172: day 1 of week 17 of year 1972. Manufactured in April of 1972

3074: day 3 of week 7 of 1974. Manufactured in February of 1974

1207: day 1 of week 20 of 1977. Manufactured in May of 1977.

Observations on character stampings:

“1” is sometimes stamped sideways and sometimes stamped upside down.

“8” is sometimes stamped upside down.

Sometimes the data code is stamped with very slanted characters. The digit “1” is often confused for “7”.

The forged dropout stamping sometimes have the date code numbers and the uniqueness numbers stamped in the same place and the digits are intermixed.