Gentlemen, thank you very much for your commentaries, even though they have my 'theory' go the same way with the pheasants that passed over these old chaps on a drive
All right, here's what I thought.
You have a bunch of guys shooting big time, and obviously concerned about their performance. Naturally, if you want to know your hit percentage, you want to know how many birds you shot with how many loads. Both numbers were too big to just remember.
The first part was easy, however, as the birds were collected and counted by servants and keepers, and the number each gun shot was announced. But how do you know how many rounds you fired?
For most of us here, it's easy: just deduct the number of shells remaining after the shoot out of the number you had before the shoot. But for these old chaps that wouldn't have worked so easily. They shot so much that even the loaders didn't help, ammunition had to be carried from one drive to another by means of a horse-drawn cart. I expect it wasn't unusual for the cart to carry up to 10,000 cartridges, just in case.
The shooting books of the period, by the way, stress that if you shoot by yourself, you can use just about any bloody gauge you fancy, but if you do driven game shooting, forget about anything other than 12. It just made things so much simpler when everyone shot the same ammo. To avoid the delays caused by servants looking desperately for those 3" 17 bores Lord Something shot, and confusion for Duke Something-Else's loaders which got Lord Something's ammo.
The downside was, however, that you never knew how many boxes of your ammo were accidentally given to other guns' loaders and vice versa.
But, if you shot a true rare bore, like a 14 (and if you're a prince, people to whom you cause discomfort aren't likely to tell you about this), than keeping score of your fired shots became much easier. You could just do a little math after the shoot, knowing that every fired shell went through your gun (and nobody would pick up your shells, either, since nobody needed this rare bore).
Edward VII didn't strike me as a person who would do such a thing, too much of a gentleman, and probably didn't care so much. I suspected George V, because he apparently took shooting more seriously than his father, and was definitely more concerned with his scores. You don't get to Top 12 of the country's best shots if you don't care about your percentages. And, I never thought that this would be anything more than an experiment, probably carried out for one or two seasons, and then discarded.
After all, if you're a prince, you could just order your secretary or some such to watch your shooting with a notepad, ticking every shot and every hit.