This product is a modified version of the Yankee Boogle to have select fire functionality without modifying the receiver or FCG. It is designed to function with Milspec pattern AR receivers and FCGs. The Choosy Express does not fit with lowered shelf lower receivers nor M16 FCGs. The files included have a few options for the selector that change the lever design.This is a simple replacement for the standard safety selector. Simply install the Choosy Express into the lower receiver with the rectangular notch towards the rear and facing upwards as one would with the Yankee Boogle or a DIAS. Thank you jimothy.perkins for your hard work on these parts!