42 magazines?
 

Why were there no serial numbers (gun matching) , on the 1942
"Black widow" mags?

Mag production had improved to the point they were all interchangeable.

Mauser stopped numbering the aluminum bottom military magazines in the last half of 1941. So they continued that policy in 1942 with the black plastic bottom magazines. Some black plastic Ku magazines were numbered.
http://luger.gunboards.com/showthrea...magazine-study