I think you are looking at a long range 'boar rifle', one built to be shot from the stand depicted. The stand probably had a locus some distance from the woods or perhaps in a quite large meadow surrounded by forest. I agree that the sight may be a type of range finder, in that the projectiles striking point from the stand was probably known for each graduation through testing. It would be most interesting to learn who commissioned the rifle. It looks like E. Bond had regulated, or at least built the sight blades to be filed to 400 yards. I expect there was some serious trial and error shooting taking place before it was used for long range boar shooting from the stand, but that the owner did just that. This of course is speculation on my part, but looking at the rifle its hard to come to any other conclusion.
The fences are as nicely done as any I've seen for function, lovely simple execution. Looking at how the action is chiseled, I think it unlikely that it was ever a flintlock.
Long range rifle game shooting has always been an area of interest & I think the person who commissioned this rifle had the property, the means and the interest to seriously pursue it. Just my thots.
