I think you are correct Kyrie.
The best answer to "What load should I use in my vintage double?" is "What loads did the maker recommend?"
It was alot easier when the makers provided those recommendations, and the shell makers provided pressure data
And this was probably part of the problem; too many choices
Not many of those powders were around after WWI
42 grain = 3 Dram Equivalent Bulk Powders
“E.C.” No. 1
“Schultze”
DuPont Bulk (40 gr.)
Curtis & Harvey “Amberite” (40 gr. on introduction - later 42 gr.)
Sporting Smokeless Powder Syn., Ltd “Cannonite Shot-gun”
Smokeless Powder Co. “S.S.” (Smokeless Shot-gun) (43 grains)
Cooppal & Co. “Cooppal’s No. 1”
Louis Muller & Cie S.A. “Mullerite No. 1”
United States Smokeless Powder Co. “Gold Dust”
Dynamit Nobel’s Troisdorf Powder Co. “Troisdorf” (41.5 grains)
American Wood Powder
“J.B.” Powder
36 Grain = 3 Dram Equivalent Bulk Powders
New DuPont Bulk (37 grains)
Laflin & Rand Bulk (37 grains)
Hazard Powder Co. “Blue Ribbon” (37 grains)
“E.C.” No. 2 (Improved)
“New Schultze”
33 Grain = 3 Dram Equivalent Bulk Powders
“New E.C. (Improved) No. 3” (introduced in U.S. in 1904)
Curtis’s & Harvey “Diamond Smokeless” (1903)
Red Star (New Explosives Co., Stowmarket)
Walsrode Smokeless & Waterproof Gun Powder Co. “Walsrode Gray”
Louis Muller & Cie S.A. “Mullerite No. 2”
Nobel’s Explosive Co. “Empire”
“Cooppal’s No. 2” (30 grains = 3 Dr. Eq.)
Poudre Sans Fumé J - 15.8 grains/dram = 47.4 grains = 3 Dr. Eq.
Poudre M - 12.76 grains/dram = 38.3 grains = 3 Dr. Eq.
Poudre T - 11.5 grains/dram = 34.5 grains = 3 Dr. Eq.