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Expert Answer
The HK USP is not included on the TLR-7A compatibility list, however the light includes Rail locating keys for Universal or 1913 Picatinny style rails. There shouldn't be a problem mounting it to the universal mounting grooves on the USP.
I bought this light to use on an HK USP compact in .40 S&W. It fits fine if none of the included rail locating keys are used. The pistol's rail is flat without grooves so you can't use any of the keys. It fits snugly and won't fall off.