FYI that the Streamlight TLR-1 series of weapon light can be made to fit the USP's 'proprietary' rail with a quick 5 minute adjustment using a dremmel tool to the inside of the light. It doesnt change the light from being able to be mounted to any other gun, and since it is inside the clamp, you cant see the modification, meaning it keeps the look and doesnt look janky or anything. Plus added bonus that the TLR series has that 'retro' look to it that fits the USP perfectly! I much prefer to have mine direct mounted with this slight mod then using the aftermarket rail adapter, which I also have but no longer use . . . ;-)Except for their stupid rail which they still haven't updated.
Yes I had to my a rail adapter for my HK USP 40. Was not that expensive back when I bought it.