This is the popular Promag 25 round .22LR magazine for the Marlin 795 semi-auto rifle.
Finally the 795 can go head to head with 25 round 10/22! The reviews from across the pond have been very positive.
Made from a tough black DuPont Zytel polymer, it is fully functioning with the 795 last round bolt hold open feature.
As well as the Marlin 795 it will also fit the Marlin 7000, and the earlier Marlin 70* on which the 795 was based.
*Please note the magazine can be a little tight in the Model 70 series, usually fixed with some light sanding if you look for any areas with insertion marks and work on those areas, go gently.