The PH16 is a Parker-Hale aperture sight that mounts solidly to the rear of the cylinder because the cylinder is drilled and tapped and the sight screws on with two bolts, making it rock-solid, but expensive to fit. So the vast majority of MkIIIs have the standard sights. It was offered by Webley as a factory option.
The aperture can also be swung out of the line of sight, allowing the open rear sight to still be used. The idea being that you could set the open for short range, say rats, and the aperture for long, say, rabbits.