Hi Dave, 41 byf, serial number 1145y, was completed about September 1941. The magazine was produced 1n 1936 for an S/42 (2904n as a spare, indicated by the plus sign). That pistol was completed in about September 1936.
Any fxo magazine with a black plastic bottom or an unmarked aluminum would be proper for the byf.
The pistol looks good to me, but check the internal parts for a match to the serial number.
If the pistol is all matching, the value would be between $1300 and $2000, depending on the amount of original finish remaining.
Good Luck