This is the factory 5 round magazine for the Armsan RS-S1 12G shotgun (branded Citadel RS-S1 in the US).
Manufactured from a tough polymer with steel lips the magazine is fully strippable for cleaning.
Also fits Saiga-12 AK format shotguns with magazine wells on which the Armsan RS-1 is based. Works in Vepr-12 shotguns with last round lock back functions.