Nadia Neither German (11-digit) nor French (13-digit) TINs have anything else except numbers in them.
But you're right that there are some government forms in France that put a letter after the TIN. It is not part of the TIN but it indicates your marital status. M for married, D for divorced, C for unmarried etc.
The thread creator however is probably from Germany or a German-speaking country. In Germany you get a letter with your TIN that is very clear and has no letters anywhere. Just the 11-digit number: