On that Russian-language forum, the first set of pictures is such poor quality it is hard to tell anything about the headstamp. The second set of pictures, in color and with the cartridge held in a set of pliers, it is somewhat easier. Enlarged, it seems clear to me that there is NO letter “A” after the date, but rather simply a corrosion spot. I have this 1940 FN cartridge in my own collection, in excellent condition, and there is no entry what-so-ever after the numerals “40” until the next entry “FN.”
What appears to be an entry, in an enhanced viewing, seems to me to be nothing but a spot of corrosion very much like some other spots on the head and the primer cup.
I don’t read Russian, so have no clear idea of what the Russian Forum comments have to say about it.