This is the style used for Atlas, at least early,1962 to perhaps 1964, and perhaps beyond.
The one shown above may work for an ES2, or Model 99, but IMO has insufficient flow rate for a 650 Dommie or 750 Atlas.
This is an Ewarts, Pull On Main; twist 90 degree, Pull for Reserve. Actually, the second pull merely locks the tap in the Reserve position.
More info for this tap may be found at:
A thread on the Commando Forum piqued my interest to make some flow measurements on an Ewarts fuel tap as used on the Atlas. The tap is pull for Main, and twist 90 deg. for Reserve. I measured the fuel delivery to the float chamber of the Monobloc carburetor, which included flow resistance...
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