This is a Streamlight Mount known as the ProTac, model 88058.
The ProTac series expands into weapon-mounted lights with this 350 lumen light featuring a dedicated fixed-mount for Picatinny rails. It uses either a remote switch with momentary/constant on operation or a standard push-button tail switch" and can run on either a lithium battery or an easy-to-find alkaline battery.
Features:
Remote pressure switch
High, low, and strobe settings
Aluminum construction
Uses one AA or CR123A battery
