I disagree with Dennis (on this one little detail), so my opinion should be suspect!
I am almost certain they are meant to be shot from somewhere outside the shoulder joint. The deeply crescent small ones with a LOT of drop may have been shot off the bicep or even near the crook of the arm, whereas the less curved ones with a more moderate amount of drop are for somewhere up the arm to the shoulder joint, and for me the where depends more on the drop and pull length than anything else. I only weigh 240 (when I watch my diet), but I'm a little short (5'11"), so "hefty" might be adequate description. To actually put the buttplate in the "pocket" (inside the shoulder joint) would require about a 12.5" LOP
! [I know this for certain, because I have a modified modern contraption set up just that way -- set it up for my (much smaller) wife, but it fits me very well also].
I have a .50 caliber with moderate curve, modest drop (2.75"), 13.5" LOP, and 42"x15/16" straight barrel (I like a little weight out front). It is based on a circa 1800 rifle, so just a bit earlier than some of the Gillespies, but not all that different in buttplate profile. I've shot with up to 100gr. (2f) off the upper arm and never felt the recoil as anything more than feedback. It goes on my upper arm, just past the shoulder and a little above the bicep -- I think that would be the best way to describe it.