This is the GSG 23 round .22LR magazine for the GSG MP40 rifle, also fits the Blue Line Mauser MP-40.
Manufactured from a tough polymer the magazine is fully strippable and features a full length bullet window with spring tension button.
You can’t go wrong with having extra magazines, whether it’s for quick loading times at the range or in the field. Or, as a backup in case you lose one or drop one in the mud.