Vintage British motorcycles use petcocks with 1/4 inch BSP (British Standard Parallel) threads. This reproduction petcocks features all metal internals and a washable fuel filter. OEM part number is 83-2801.  Most commonly used on vintage Triumph 250, 500, 650 and 750 cc motorcycles.
Most British bikes use a main valve, and a reserve valve.  This is a Reserve valve and is intended to use on the left side of the tank.    

There is only On or Off position with these fuel valves.