At the time there were no real mods. Only had to make the new side clamps. The screw, blade, spacer and loc-washer were what replaced the original rear sight.
The clamps for the blackhawk were uneven do to the ejector rod, I copied the longer and made two new ones with a bit more bite.
But unfortunately B-square changed there design, and I no longer work in a machine shop.
Its the reason I still have this. Keep telling myself I'm going to get another GP100.
Action arms used to make one for the GP100 4" but I don't believe they still make them. Was one on the evil eBay and awhile go.