Thank you Pat,
It took me a while to find a cm ruler in my house. Your measurements correspond to my measurements as far as I can tell. Those mm marks on my ruler are very small for my old eyes. So at least I am not alone in having two different sizes of serial numbers on my two Borchardt mags. Since your larger sized serial number belongs to the smaller serial number and my serial number/number size is opposite to yours, I am assuming that the size of the serial number stamping was probably random. In any case, it sorta confirms in my mind that my newly acquired Borchardt magazine with the 2,5mm numbers is original and correct.
Here is how it currently breaks down:
Big Norm...................vs......Pat
Ser # 552 = 2,5mm...........Ser #1174 = 4mm
Ser #1966 = 4mm.............Ser # 2173 = 2,5mm
Again... thank you, Big Norm
