This is the Smith & Wesson 25 round .22LR magazine for the S&W M&P15-22.
It features a full length bullet window and an easy load pull down button. Well regarded for it’s reliability thanks to it’s strong box spring, robust easy to clean design and upwards facing rear lever, which operates last round hold open on compatible rifles.
Using M&P 15-22 magazines in other AR15 lowers?
M&P 15-22 magazines are popular in AR15 .22 rifles fitted with last round bolt release catch mechanisms, including the KRISS Defiance DMK-22 which comes with one as standard.
Please note M&P 15-22 magazines are slightly shorter front to back that AR magazines (by just over 2mm) so work best with special shims fitted.