mounting micro dot on Ruger 22/45

[Symbolic]

I would like to mount a micro dot on my Mark III 22/45, but I don't like the rail. I saw this picture and I really want to do this to mine. Does anyone know how these micro dots are mounted? I only have the 3 pre-drilled holes on the top recevier. Will another one need to be drilled and tapped for this setup?

Edit to add: That's a Leupold DeltaPoint on the top setup and a Burris FastFire on the bottom setup

Edited October 13, 2012 by Left2Die762

[volfanatic72]

Those are mounted on a special base which mounts directly in the dovetail for the factory rear sight. I know that Burris makes the mounts (maybe others?), and they are available on many of the usual websites.