I think the screws in front of the trigger guards are for fixing hand guards to protect the hands when this model was used for practicing duelling, similar to the guards on some swords such as scimitars, I expect the guards are missing, I have resealed many Giffard pistols and rifles, they are not as rare as is suggested, this model with the guards are very rare, I have worked on a couple of these, the target version of the rifles are also very rare and are beautiful guns.
Cheers, Lawrie.
By the way the valve system is very simple it involves just one seal located in the reservoir, and is very effective, with modern seals they will last another hundred years, I wish modern air guns were as simple.
In some old Giffard adverts they show the duelling versions being used with what looks like heavy leather floor length coats and mesh face masks for protection, from an opponent.