The Atrius Development Group FRS X-Series is a universal ambidextrous selector switch designed and manufactured in the USA specifically for Sig Sauer gas piston platforms, including all generations of the MCX, MCX-Spear LT, MCX-Rattler LT, MPX PCC, MPX-K, and the MCX-Spear LT Upper AR Conversion Kit. Constructed from heat-treated 4140 steel with a corrosion-resistant black nitride finish, this component features a distinct "Safe, Semi, Super-Semi" configuration and includes a newly dimensioned FRS (Forced Reset Safety) lever optimized for seamless integration without modifications. While it is broadly compatible with the single-stage mil-spec triggers factory-installed on MCX and MPX platforms, it is strictly incompatible with the MPX Copperhead, Geissele triggers engineered for the MCX/MPX, cassette-style drop-in triggers found in select MPX models, and specific Sig factory units like the Two Stage Match Duo AR trigger or skeletonized AR triggers that feature a rear U-notch or relief cut on the trigger bar.