I have a B Square mount for a 357 that I like very much, but it is a little different from your picture. Actually I have two Ruger 357 Blackhawks one with a Red Dot and one with factory iron sights.
The B-Square I have requires you to remove the rear sight screw and stuff and it screws into that for its rear attachment. The front of the rail has a slot in it that fits over the front sight and what looks like a set screw from the side that helps hold it on to the front sight.
At first I was concerned that it might not stay in place, but the zero has not moved at all (at least for me).
Part of why I went with the B-square was it was available locally at far less than the manufacturer's suggested price, it didn't require any drilling of the Ruger, and it was in stainless to match the stainless Ruger Blackhawk I have.