I trained at a British quad 50 school at Langham Camp, England (UK) in 1956 (yep, I'm an old fart). The U.S. Air Force used them for Air Base Defense. We got to shoot at ARCATs over the North Sea. I say "shoot at", because our British instructors would hit us on our helmets with a small club if we looked up from the sights. There was so much smoke coming off of the guns I couldn't see anything through the sights.
We found out later that they were running out of targets, and didn't want us to hit any more. They were a small aircraft out over the water, and when hit would parachute down and be recovered by boat.
There was a quad on a trailer in the Atlanta area a couple of years ago, that I think ended up being purchased by someone in the Chicago area.