The quarter rib cut to accept Ruger Rings. The rings attach directly to the quarter rib. Several manufactures also make rings to fit the rib on Ruger rifles.

Some find this put the scopes a little too far forward. I've never had that problem and I use Leupold scopes almost exclusively. You do need to use a scope that has a lot of eye relief, even if it's not Leupold.

If you find your scope for some reason doesn't work mounted that far forward for you, SK mounts makes a mount that does offset a little farther back. I've used SK's skulputured rings on my No.1 RSI and they seem very well made and easy to install.
http://www.scopemounts.com/ruger.html