MagPump 9mm Double Stack Magazine Retainer for H&K USP P8 Black MR-H&K USP P8-03. Your purchase of the 9mm Luger includes a set of six adaptable retainers to fit pistol models in SIG Smith & Wesson Springfield Armory CZ and Ruger. Retainers are designed to fit double stack magazines for popular 9mm models. Specifications for MagPump 9mm Double Stack Magazine Retainer: Manufacturer: MagPump Color: Black Package Contents: MagPump 9mm Double Stack Magazine Retainer MagPump MR-H&K USP P8-03: 9mm Double Stack Magazine Retainer for H&K USP P8 Black