From 1897-1903 the Belgian makers kindly told us exactly what load, and powder, to use

12g Manufacture Liegeoise likely proved with Mullerite (stylized lion over M) for a service load of 2.70 grams POUDRE = 41.67 grains = 3 Dr. Eq. (42 grains) Bulk Smokeless Powder.
"E.C." No. 1 & "Schultze" were also 42 gr. = 3 Dr. Eq.
36 grams PLOMBE = 1 1/4 oz. Shot

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and for us dumb Americans even made it simple; a William Read & Sons tradename gun

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The Belgian Proof House specified service loads in 1906

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12g - about 1 1/4 oz. / 3 1/4 Dram Eq. (1220 fps).
(12g 5.8 grams = 89.5 grains = 3 1/4 Dram)
16g - about 1 oz. / 2 1/2 Dram Eq. (1165 fps).
20g - about 7/8 oz. / 2 1/4 Dram Eq. (1155 fps).

After the proof house revisions of 1924, a Certificat d'epreuve was issued and specified pressure, not the shot or powder:
"The pressure developed, measured by crusher type device , lower or equal to 600 kg per square centimeter for sizes 16, 12, 10, 8 & 4; 670 kg pounds per square centimeter for sizes 20, 24 and smaller."
600 kg/cm2 = 8534 psi SERVICE pressure;
670 kg/cm2 = 9530 psi SERVICE pressure
+ 10-14% by piezoelectric transducer measurement.
20 g was PROVED at 1000 kg/cm2 = 14,223 psi
12g PROVED at 900 kg/cm2 = 12,801 psi
Transducer numbers for 20g would be close to 15,500 psi; 12g about 14,000 psi

I don't know if any maker's catalogs thereafter included load recommendations

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