I get the feeling that the person who put together this particular example above goofed and screwed the grips onto the opposite sides of where they should be. The top edges are angled differently on most handguns.
It definitely looks like something that could be scratch-built.
I'm thinking the trigger assembly could be built with a MDF core with sheet-metal cladding. Around the edge, use a really thin strip around the grip, and use a really thick piece of sheet metal for the trigger guard.