When I was sworn in as a rookie police officer in 1975, we still had one of those things attached to a model 870 Remington 12 gauge shotgun. I'm not sure if anyone knew even then what it was for. I think that the hand thrown tear gas grenades fit into it. I know that we also had some black powder blanks, and now that I'm thinking about it after not seeing one for so many years, I think that we may have even tried one out. If I remember correctly, and that's a big if, I think that you pulled the pin on the grenade and held the spoon in place and slid the whole thing into the sleeve of the device you have. Then when the blank was fired, the whole grenade was launched, and as it exited your contraption, the spoon flew off, and the grenade was propelled towards the target, and you hoped it didn't hit anyone on the noggin, or it would probably kill them.
I think that it could be used to fire cans of beer to officers who became 'pinned down', and we're dying of thirst??? (Just kidding).
Wow... That brings back memories. I became a firearms instructor in 1977, and had access to all kinds of stuff that was in the department arsenal. We had m16s, and some really cool smith and wesson model 76 9mm sub machine guns. Had all sorts of now defunct tear gas and 'sickening gas' devices, most of which were obsolete even then.
Here's what probably fit in it. Just found this picture online. Ours looked something like this, but this one is labeled "Smith and Wesson", how cool is that?