I just received my SIG Pro-cut P229 slide. I already have an RMR, so want to mount that to the slide. None of the screws I have will fit (all being, I believe, 6-32 that come with RMR). What are the RMR mounting holes threaded at on the P229?

If it's the same as SIG's other offerings, it's the fine thread version of #6 fastener, which would make it 6-40.

I've pretty much narrowed it down to that. I had read either M4x0.7, or 6-40. I found a screw that was M4x0.7, and it fit the holes for the Romeo optic perfectly. But won't even start in the holes for the RMR (which are smaller). Since it isn't 6-32, I have now come to the conclusion that it is 6-40. I should have about a hundred of them showing up today, so will post back on here as soon as I can after checking if two of those fit. If they do.... I might have a bunch of the proper sized screws to sell in the near future.

Now I can confirm that the screws needed for mounting the RMR on the P229 Pro-cut slide are 6-40's Specifically, in my case, 6-40 x 0.5".

With the RMR mounting plate, the 6-40 x 0.5" worked for both sides, and does not seem to interfere with the extractor on the right side. Prior to mounting, I noted that I could press the back end of the extractor and get it to flex out. After mounting the RM04 and snugging everything down good, I tried pressing the extractor, and got it to flex out just as easily as previously, and I couldn't feel it impinging on anything.

As far as I know, the RMR mounting plate is not needed for the RM04; however, I used it anyway, as a little of the RM04 hangs over the side of the slide. Plus, I was hoping that just that little bit of additional thickness would ensure no interference with the extractor. I did not try it without the mounting plate.