Vintage/Antique RADIATOR KEY with Letter P Opening is size 6 on US scale   I LIVE IN MY GRANDPARENTS FARMHOUSE AND IT IS 200 YEARS OLD.  THIS KEY HUNG ON THE WALL FOR A LONG TIME.  THE RADIATORS IN THE HOUSE TAKE A SMALL KEY SO I DIDN'T KNOW WHAT THIS KEY WAS FOR..UNTIL I LOOKED IT UP AND IT IS JUST ANOTHER SIZE KEY FOR DIFFERENT RADIATORS AND IT IS NOT FOR MINE.  MY GRANDMOTHER SHE LIKED SAVING THINGS....GUESS I'M JUST LIKE HER!!