This Brass High Pressure Part 1 Ball Valve 1/2" is used in toilet cisterns and cold water tanks, with a cistern float attached. A part 1 valve has the water outlet spout on the underside of the float valve.
Brass Ball valves often found in loft cisterns, such as the cold water storage cistern and the feed and expansion cistern. Working in similar ways, as the cistern fills and the water level rises, the float pushes the float arm up, which moves a rubber washer against the valve seat, closing the valve.
You can identify which one is fitted currently , by inspecting the point of water leaving the Brass Ball valve.
On Part 1 or 'Portsmouth' valve, the outlet is on the valve's underside. Whereas on Part 2 or 'diaphragm' valve, the outlet is on the top of the valve. Water flows into the cistern via a small plastic spout which screws onto the valve body.