The Magpul STR (Storage/Type Restricted) Com-Spec model is a drop-in replacement buttstock for AR15/M16 carbines using Com-Spec sized receiver extension tubes. A storage-capable version of the CTR®, the STR has improved cheek weld and two water resistant battery tubes on a compact, reinforced polymer A-frame design. A shielded operation lever and supplemental friction locking system prevent accidental operation and minimize excessive stock movement for enhanced weapon stability and accuracy. The STR incorporates two integral sling loops, a reversible push-button QD sling mount, and a replaceable 0.30 inch rubber butt-pad which provides positive shoulder purchase to prevent slippage even with body armor or modular gear.
Unsure whether Mil-Spec or Com-Spec is required? Link