This issue gets debated every few months, and it's possible this 'upright head position' idea may be a myth. Look through the early 1900s posters, calendars, and die cuts shown in
Classic Hunting Collectibles by Hal Boggess, esp. the Trap shooters on p. 47 and p. 80. To my eye, they all have their cheek firmly placed on the stock with their neck extended far forward; or 'crawling the stock.' A pinback on p. 208 shows a trap shooting lady and it look like her nose is actually touching her right hand. I had a 1928 Ithaca catalog with pics of champion trap shooters and every one, at least in the pics, had that same head and neck position, much different from the upright head position favored by trap shooters today and made possible by long LOP and adjustable Monte Carlo combs. On p. 76 is an actual photograph (rather than an illustration) of men shooting skeet using very similar gun mounts as would be seen on a skeet field today.
Wing shooting from
American Game-bird Shooting by George Bird Grinnell, 1910
A. B. Frost

Trap illustration

Trap shooting photo (obviously posed)
Neck extended forward, head scrunched down on the stock, eye looking straight down the rib
Here's a very interesting Peters die cut showing a man in a shooting position but without the shotgun-clearly head down and neck extended.

And another Western bird shooter

Interesting photo showing the two shooting styles from "Modern Shotgun Shooting" by Lawrence B. Smith, 1935
Bottom line:
1. There doesn't seem to be alot of difference in the gun mount for
wing shooting comparing illustrations from the early 1900s and shooters today. Rather than standing erect with a straight head and neck, early 1900s shooters 'crawled the stock', extending their head and neck far forward (probably touching their thumb with their nose?) One can imagine much less perceived recoil with the head forward and firmly on the stock, rather than lightly touching toward the heel, with the gun levering upward. When we try to shoot these 'lotsa drop' old guns, in order to look down the rib, we lightly touch the comb lower on our face, or even along the jaw, rather than under the zygomatic arch/cheek bone. Doing so just accentuates perceived recoil, especially to the face. And I shoot my 3" drop LCs by 'crawling the stock' comfortably and am
very recoil sensitive.
2. Certainly
trap, and to a lesser extend, skeet gun mounts have changed with the pre-mounted gun.
3. To hit the bird/target, we need to be looking where the gun's shooting.
4. In order to do that, we must look down the barrel somehow.
5. More drop is 'built in' cast off ie. you must mount the gun a bit lower on the face moving the position of the eye to the right (for a right hand shooter.)
6. We seem to be made so we can adjust pretty well to about any gun, once we figure out where to put our head

I can't 'prove' any of this, but it's interesting (and possibly illuminating) to look at these old pics and illustrations.