OK I did some searching on anatomy.
The shoulder "pocket" is formed between the deltoid and pectoral muscles when the elbow is raised to shoulder level.
Here is a "Gov't spec" shooting coat. Bought it at a yard sale. Recoil pads, pad for the sling, elbow pads.
Note the recoil pad extending onto the arm.
In this case the "pocket" is about centered on the rectangular shoulder pad. Between the deltoid and pectoral.
Yes the pectoral is really in this area but raising the arm forms a pocket between the deltoid and upper rib cage.
This is where the buttstock is supposed to go when shooting a shotgun or shooting a shotgun butt rifle when prone or sitting or kneelling. Offhand is different and in most disciplines results in a stance that is a modification, if not identical to the old schuetzen stance with a "hook" under the shooting arm and the non-shooting elbow braced on the side of the chest.
As I previously indicated the NMLRA, at least SFAIK, does not allow this. So the stance in NMLRA offhand is somewhat different.
The NRA BPCR silhouette rules allow any stance but ban deep crescent buttplates, 1 1/2" is max IIRC.
So we have different disciplines, different shouldering for different shooting positions and different peoples use of those stances.
I will state this. It is very difficult to win an offhand match shooting your rifle like a shotgun if shooting against people using the Schuetzen stance, all things being equal its impossible. I suspect this is why its is or was banned in NMLRA competition. Someone likely used it to out shoot some folks in a position to make such rule changes.
It is proven by the fact that most offhand disciplines use a form of this stance and shoot off the arm.
This stance also allows the use of a heavier and thus most stable offhand rifle.
For those that doubt that the schuetzen stance is more accurate put a 8x-10x scope on a rifle and then test various stances for "wobble" on a 50ft 22 range. I used to shoot 19th century SS 22LR at 50 ft as practice for BPCR silhouette in competition with others doing the same thing. Stance, just a seemingly minor difference in how the feet are placed, can make a considerable difference in accuracy.
Also a 100 shot 200 yard offhand match is easier to do with the schuetzen stance. But this is not often done with flintlocks.
I would also point out that most "American" rifles prior to 1790-1800 had pretty flat plates.
And finally its one of those Kawasaki "Different strokes for different folks" things.
But you 40 and under types probably won't remember this slogan
Dan