https://en.numista.com/catalogue/images/5eaf07df5fe02.jpg
They call it letter + alphabet number in this translation but they are only numbers in the alphabet part. So it has to be a term like block/ contol number or similar but still quite confusing.
Edit:
Seems to be specificly French (LINK)
3. Spec., tech jobs.
− FIN. (bank). Issue by alphabet. ,,Mode of issuing bank notes, according to which the Bank issues its notes in batches of 25,000, each thousand corresponding to a letter of the alphabet, A, B, C, etc., which is written on the note with a number indicating how many alphabets have been issued; each alphabet comprises, as we see, 25,000 notes; and C329 placed on a thousand franc note expresses that this note belongs to the 3rd thousand of the 329th alphabet.`` ( Littré ).
quite a convoluted way 😅
J - 15th place in the alphabet (probably should mean million not thousand in the translation)
633*25000=15825000 [15808895 control number]