My bad, I had NP27 on my mind which is a different gun, 1911.
I don't know about milling the slide on Sigs; some one here may?
If you measure real close, I could look at that mount I have, It is a Cadex P226 red dot, you may find some specs on the web, my good calibers are buggered.
have crappy ones, they never get hurt for some reason?
I have a Sig 220 45, don't have the other one any more, but the sight should be the same be the same, but never did much with these models