Hi Roly,
As Troubledshooter says, PH 16M and 17B aperture sights were a Webley stock item that could also be fitted to standard Mark 3s. Parker Hale catalogues also stocked the FS22 foresights as an option on Mark 3s, so your rifle is not all that unusual. Indeed Guy has recently sold an early Mk3 with both aperture and tunnel foresight and I have a couple in my collection too.
John
Currently looking for Baikal Makarov pistols with the following prefixes to the serial number: 98, T01, T09, T21, T22
Prefer boxed or cased but will consider loose examples too.