Found my keeper - byf 41

[Nomadr]

The suffix is part of the serial number. They ran in blocks of 10,000. The first 10,000 having no suffix, the second 10,000 would start with the serial number followed by "a" and so on through the letter "x" .

Yours without the suffix would be late in 1941 and would explain the fxo black bottom mag.

Bob