This mag was produced by Mauser and is stamped with P.38 on the left side of the housing.
The spine is marked with two Mauser acceptance stamps E/135. The top E/135 is stamped up side down. Double acceptance stamps are only found on very early Mauser magazines and were issued with byf42 pistols with no letter suffix and very low serial numbers!
Only a very few of these double stamped Mauser magazines were made and this magazine variation is therefore very rare. The smaller internal parts are not stamped with E/135, but this is sometimes observed.