Kwik Site vertical rings. They are cut from a continuous bar of extruded aluminum, therefore both halves are identical and they eliminate the need for lapping rings. I'm slowly converting all of my mounts to this set up. I'm a ring lapper, but I can't find any reason to with these rings, they are perfect, not to mention dirt cheap. They are what I use on my .454 Casull 12" Encore barrel and they've held up great for several years now.
Selmer