The URX appears to share the frame with the Challenge, and has a straight front strap. Guns with the UDX grips have scallops in the front strap that correspond with the finger grooves of the wood grips.
Browning is producing fewer and fewer models with what I call the "regular" grip frame. That's the one they've been using since the Buck Mark was introduced. The first oddball frame was for the Challenge and it was advertised as a frame for smaller hands. Recently what appears to be a Challenge frame is being used on the URX models. The (shorter, front-to-rear) frame is used because the URX grips wrap around the front and increase that dimension. More recently, they began offering the wooden-gripped UDX that has those grooves/scallops I mentioned.
Few aftermarket grip makers are offering grips for the URX, UDX and Challenge. Someone on another board mentioned that Tactical Solutions is coming out with aluminum grips, and I think there's at least one other manufacturer who has something.................chim