I might know what that is, Back in the late 1970's and early 80's there was a company that sold a item much like that, it was for putting a 2 leter bottle on the end of a gun, making a ONE SHOT supressor. you wold have to replace the bottel after each shot as the bottom blew out. I actually heard that they worked. check it by trying to thread a plastic bottle on the end with the mouth fitting inside of the recess. it should screw in, if it does, then you know what you have .