I have a flapper that rest inside a brass cylinder which sits on top of the opening at the bottom of the tank. It keeps running and i wish to replace it or fix it. the hardware store has never seen or have i, a flapper like this. i was told this toilet is from the 50’s. the tank and bowl on one piece so i can’t see what hold the cylinder down. doesn’t someone know what this is called or where to find some information on this type of flapper?