Yea this is a brand new 584. So im not sure who changed their tolerances but I have a feeling it was Ruger. I know they stopped producing the Mini 14 during Covid for a while. Maybe these are new manufacturing machines or something.
Thanks for posting this, I noticed because of your post, I have the same issue and this could be why I'm having trouble getting the sight aperture low enough, I'm bottoming out on the adjustment by the time I get a zero. I didn't notice the gap being there when first fitted, but now checking it after seeing your post, there it is, I guess it was when I tightened the screw down that it opened up and I just didn't notice. Mine is a 584, but close to 2 years old now. The sight was originally a tight fit on the receiver, I had to push it hard to the front to get the screw to line up, so it would go in, it didn't occur to me, it could move as I tightened. It's loosened up of late, I think the side grub screws have stretched it out a little under tightening. I made a shim from a very soft piece of stainless steel I had in my garage, this dropped the rear down quite a bit to sit on the receiver, so hopefully I will now have the elevation adjustment, without it bottoming out. Now I really look at the sight it is an ugly fit to the receiver, not really impressed at all for the price.