This classic image of John Noveske had me wonder what he is holding. I found out it is a Mako product called the UTA (Universal Tactical Attachment). The giant hook has an integrated picatinny mount so you can mount the Glock onto the rail of a long gun. It is an adapter that plugs into a Gen 3 Glock grip. It reminds me of the ENDO Glock stock adapter but it does not have any upper support on the beavertail. So I imagine the Glock will move a bit under recoil and muzzle climb.
I think the A3 Tactical double picatinny clamp is a better solution when using a frame mounted rail setup. There should be less movement when the Glock is fired.