This mag was produced by Mauser. The only marking that is visible from the outside is the Mauser acceptance stamps E/135, which is located low on the spine. There is no P.38 marking on the left side of the magazine, as typically seen for other Mauser magazines produced by Mauser. These were issued with byf42 pistols with no letter suffix and very low serial numbers!
Only a very few of these Mauser magazines were made and this magazine variation is therefore considered extremely rare. The smaller internal parts of this specific magazines are not stamped with E/135, but this is sometimes observed.