You may find that the rear sight is attached with a screw underneath the windage slide.
Farquharsons have a screw right where your sight is and there isn't much real estate underneath to put screws coming from the underside especially on a top tang safety 3-C action.
Maybe adjust your windage all the way out and see if there is anything underneath it.
I am thinking that the action was originally used on a complete British rifle and not brought into G&H as a new action. The engraving and finish on the action is very typical of the British trade Farqs.
