This is the GSG 10 round long format .22LR magazine for the GSG StG44 (Sturmgewehr 44) and Blue Line Mauser STG-44 rifles.
The same size as the 25 round type restricted for 10 rounds for use where 10 round restrictions are in place.
Manufactured from a tough polymer the magazine is fully strippable and features a full length bullet window with spring tension button.