These are cup-style grenade launchers that attach to a Lee Enfield No1 Mk3, used for launching Mills Bombs via a blank round. Virtually unobtanium these days unless you want to significantly overpay. The only store that I saw had these in stock was charging $400.

1 works perfectly, the other 3 have minor/major issues that can be solved if you have a welder or are handy. 1 of them doesn't lock close on the screw, and the others have stuck screws. Easily fixable with a little knowledge and elbow grease. 

$150 for all 4. You can easily make your money back by fixing up and reselling 3 of them and keeping the one that works. 

Pickup near I10 and Cortaro