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 main valve and is intended to use on the right side of the tank.  The left side reserve valve and the right side main valve usually connect at the carb or banjo fitting.  The reserve valve would be in the off position until additional fuel is needed.  

there is only On or Off position with these fuel valves, no reserve.