If your staring at your radiator and you know for a fact that it is a one-pipe steam radiator and you do not see an air vent, then you probably have one of these. This is the invisible air vent that came out in the late 20's and was really quite a masterpiece of engineering. It made the radiators look much nicer, but that wasn't its real intention. The reason for its existance was to provide better heat out of the one-pipe radiators by eliminating the air pocket that would be left in the radiator when a standard air vent was used. If you have radiators that have these and a radiator is not heating, check to see if a painter painted shut the little air vent hole. If so, poke it open with a paperclip end or similar small wire. By the way, sadly these are no longer made.