Well, I called Ruger and gave my serial number with which they found the model number. After a short discussion I was told they could have someone more knowledgeable call me back. I agreed.
I did receive a call back. We talked about the situation. The 'XRP' on the inside of the stock grip panels was explained as a possible typo where the 'P' should have been an 'R' or some such, which seemed strange but okay. I was then told that he had a monogrip and a SBH of the same model with him, and the grip fit his, so it should fit mine. He didn't have much response to Eagle Grips selling the XRP grip panels. But he said they didn't have an XRP frame, and that mine had to be an XR3-RED.
He told me that installing the monogrip was tricky, and about the issues he'd had fitting it, but that it did fit. That the edges of the grip bound up at the back strap, and that's what happened to me. I got the impression he thought he was talking to an idiot who didn't know how to put a grip on. But it turns out he was in fact talking to just that idiot....because I went and bought another one, and after a long fight pushing, pulling, and bumping, with some prying and gouging of the grip edges, along with shearing the internal nylon frame of the grip, I did finally get it on.
Sorry for wasting your time, Varminterror. But thanks for all the researching, I appreciate it.