This is a high quality all steel 5 round aftermarket magazine for 1930s and 1940s historic Mauser MM410 and MS420 .22LR military trainer (Mini K98) and sporter style rifles.
The magazine has an angled base and fits flush to the machined trigger guard, which have distinctive button operated magazine catches on these models.
Not to be confused with later 'B' versions with curving magazines mounted forward of the trigger guard.