Steven, an un-numbered aluminum base fxo is also correct for your no suffix 41 byf.
Mauser began serial numbers in 1934 with 1 no suffix (NS). At S/N 9999, they began 1a S/N and continued through the alphabet, skipping the letter "J". In late 1937, after 9999z, they started over with 1 NS. In early 1940, they started over again, then finally near the end of the 1941 production the started over again.
Hope this helps.