The Parker-Hale 16m was the sight fitted to the Webley Mk.III as a factory fitted option. It shares the same fixing base as the 17b as fitted to the Supertarget, so either can be fitted to a pre drilled action.
The PH 16m is preferred by many as it is a smaller sight and can be swung out of the way so standard sights can still be used.
There are many variations of the PH 16 sight, but only the 16m variant can be fitted to the Webley Mk.III.