Mc, the current iron sights are reported to be their old proprietary sights which are those shown on the one in the H&R pic, they were using plastic sights and Williams WGOS and now have gone back to the old style like this, the elevator adjustment screw is loose on it since it's not installed.
None of those are like the carbine sights, it's a different style with a stepped leaf elevator, I believe.
Sight mount screws on my .500S&W are 6-48 as near as I can tell, they should be the same as the scope mount holes.
Tim