Rob, yes, exactly this part. It seems that it's just some sort of pin which fits in a corresponding hole in the action.
I guess in yours the screw fixes the sideplate to the action, no? Since most of the 7pin sidelocks are copying the H&H royal, I was somehow surprised to see this deviation. But actually it seems that also H&H uses both variants:
http://www.thefield.co.uk/shooting/holland-holland-38434 It seems that the pin and the plate are machined from one piece, at least it does not look press-fit to me.
Still makes me wonder about the reason for having both variants from the same maker and in the same time frame. Is one version somehow considered better? Also it looks that (at least not all) locks from Francotte were sourced from Jacquet. I thought the company was large enough to produce their own locks.
It's all not really important but sometimes guns can tell interesting stories when you look at the small details.