It's a pressurized lid from an old pressure relief or a boiler steam condensate valve. The hole in the top accepts a screw down point to seal it into the valve seat. As it is compressed into the valve it compresses a spring that is activated by a float. When it fills with water, the float pops up the spring compresses, the valve opens, and steam blows the water out. When it's time to clean it you just remove the hold down, pop this cap and the spring and valve beneath it lifts out. I have lids like this on different items also but this one has the screw hold down. Basically think of a U shaped wheel puller holding it down. Kind of like a valve spring cover.