I have an R31 that I bought back in 1993. I don't remember having to drill and tap for a scope mount but that doesn't mean I didn't do it. Like you, I have an ancient Aimpoint mounted on mine. I just turned it on and it still works although it seems a bit dim, probably needs new batteries. Haven't shot it in years. I had to send the Grendel back once or twice as it would double every now and then. That would have been a great feature if it would do it reliably, but it was a sometime thing.
Its successor, the KelTec CMR-30 I bought in 2016. I added a red dot sight, and the slanted foregrip because the rail is uncomfortable to hang on to. I should take them both to the range and compare them side-by-side one of these days.
