​16mm heavy brass cylinder with black rubber plug in one end and rectangular slot in side

Found digging in an old flower bed in the North of England. Some sort of dashpot/thermostat/spring cartridge? The remains of the rubber plug are to the left in the photo, it had bulged and split. Doesn’t seem to be anything under the plug except a flat metal surface. Part of a ballcock, perhaps? Found amongst pieces of porcelain that may possibly have been rubble from an outhouse.