Didn't quite fit - Streamlight H&K USP Full Size Adapter for TLR Gun Mounted Flashlight

Unfortunately this is your only option if you want to mount something to an older USP. Got it on eventually, but it is slightly too small and the rails don't align perfectly, which many effect alignment slightly. Almost like it was machined a tiny bit too small
Pros: Good in theory
Cons: fit was just slightly off

This review was written in the old system and had content requirements that are different than reviews written today.