What's with this S/42 magazine?

[DonVoigt]

The "42" indicates a replacement magazine.
Would have been "unmarked" when new and in the system; marked somewhere along the line.
Could be legit or not.

Lots of folks "like" the unnumbered mags with the 42 or s/42- and are not above "adding" a number to make a "kind of a" match.

In your case for a mismatched pistol, it is neither here nor there. JMO.