I also had what I tought was a 1940 byf earlier. That was my first Luger. I got it cheap and it turned out to be from 3 different guns (with some good help from this forum). I traded the gun + 3 mags for this one.
All numbers apear to be matching except magasine.
I quess to match the four digit number on the front of the slide from a different gun with the frame number from byf is hard, but never know what have been done with these old guns during the years.
Think I can say for sure that the numbers are not remake or fake. The stamps on right side also give the impression that its a byf. Since the price was good, I'm not going to <img border="0" alt="[ouch]" title="" src="graemlins/c.gif" /> if it turns out to be from two guns, but I hope it's from the same Luger. Thanks for the note btw!