THE LARGER HAMMER BASE DIAMETER
In about 2000, Ruger made a slight change in the rear of the main frame where the 'ears' of the grip frame on both sides of the hammer mate to the main frame.
The Bisley grip frame ears are taller because of the way the two Allen head mounting screws are internal. So the main frame rear mating surface is about 1/16" taller then the main frames made for the standard plow handle grip frames. So around 2000, to simplify production Ruger made all their main frames with the bisley size rear mating surfaces. Therefore all plow handle grip frames from then on have taller ears to match the main frame. But ever since then, the hammer base diameter is a little too small to have a nice flush fit with the grip frame ears like they used to.
However new design hammers since then and there's only three, (the new vaquero long spur and Montado, and the Flattop Blackhawks) were designed with slightly larger bases to once again, fit nice and flush with the larger grip frame ears.