Often under the balance, sometimes under the dial, there is a letter and numerical code representing the month and year of production of the movement (or at least the plate).
One letter for the month (A = January, B = February, etc.), one or two numbers for the year. That is, a watch made in February 1962 has the stamping B 2.
Since the year is known to have twelve months and the L is the twelfth letter, would have to be end with the L. To avoid confusion with the letter j or the number 1, the letter i was omitted, the letter M was used as the month of December.
The J was used and stands for the ninth month, ie September.