The early, diaphragm type, steam valve. The overflow pipe on the left sits inside the main body with its upper edge sealed by the centre stud of the diaphragm. The spring in the hollow centre nut exerts a downward pressure on the other side of the stud. If radiator pressure increases, due to over heating, the diaphragm is forced upwards allowing steam and water to evacuate down the overflow.
See 19th August for further details.