Interesting hypothesis brother; that France would require use of French powder for proof for shotguns exported to France.
I looked through this document from 1906 regarding the Banc D'Epreuves Des Armes a Feu De Liege and unfortunately there is no mention of that possibility.
https://books.google.com/books?id=5fxGAQAAMAAJ&pg=PA53&lpg
The gunmakers could specify the powder to use for the Fourth “Optional test with smokeless powder”
After 1898, “Poudre Vive Unique” proof powder was adopted, and used unless otherwise requested, at extra cost.

As you noted, November 18, 1903 a decree was issued that the 4th (Nitro) proof (no matter the powder chosen) was not to exceed 875 Kg/cm2 or 13,000 PSI (about 14,500 PSI by transducer measurement).

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