This is a GG&G, Inc. Mount known as the M3/M6, model GGG-1133SP.
With the large number of flashlights and lasers that fit on a Picatinny or MIL-STD-1913 rail" GG&G has designed and manufactured an HK USP full size flashlight mounting rail adapter meeting the Picatinny Rail specifications. Fits tightly to the USP trigger guard allowing for the rear rocker switches to be activated by the fire control finger or thumb of the off hand.
Fits tight enough to retain zero on laser sights
Clamps onto the existing USP light rails
Solid billet 6061-T6 aircraft grade aluminum