Just one note on the G&H topmount. This is a relatively recent offering, dating from the '70s I believe. The bases are small and light and enable a mounting as low as the scope profile will allow. Repeatability is, in my experience, absolute. I have these on my favorite buffalo gun, which gets shot every couple of years on average. First time at range, the scope goes on, the levers come down, the 300gr loads the rifle was made for go in the mag, and three shots go into under an inch at 100 yards. Without fail. As far as I know this is stll an exclusive G&H offering. It used to be, maybe still is, that you could buy a sidemount and do it yourself. I don't believe that's true of the topmounts. In any case, they are, IMHO, highly desirable, more so than the sidemounts and all other things being equal, I'd prefer a G&H with them to a similar one with the sidemount.
