Mbatten;
I have considerable experience in rebuilding Stephen Grant designed ejectors, kickers, links, bridles, sidelock tumblers and so forth; and the gun pictured in the link posted by you does not have the Stephen Grant ejector system. The reason for my determination is that one cannot see the set of ejector trippers that are adjacent to the cocking arms on the Stephen Grant ejector system.
The Stephen Grant ejector system is not complicated, but it has too many wear points in the system of links, pivots and studs that comprise the system, therefore there is greater opportunity for failure of the system versus the Southgate ejector system design.
However, the greatest disadvantage to the Stephen Grant ejector system, to my mind is the increase in weight in the forearm over the same forearm if it is fitted with the Southgate ejector springs and kickers.
It is my opinion that John Robertson, owner of Boss & Co., designed his coil spring ejector system after considering how the Stephen Grant ejector system worked. John Robertson had much opportunity to observe and fine tune the Stephen Grant ejector system, as Robertson built(one of which I own) a number of Stephen Grant guns while he was a gunmaker to the trade in London before he purchased Boss & Co. John Robertson solved the overweight issue as well as the reliability of coil spring ejector systems.
If you have further need of information you can PM me and I will endeavor to answer your questions.