None of my French catalogs are that early, but, as late as the 1936 Darne catalog, and a mid 1930s( Jallos & Cie ownership) Fransisque Darne catalog were both advertising 10 gauge guns, and the Darne catalog has "Canardier" (punt guns) at the back. No mention is made of legality in France, but the load for the 50mm is noted as "120 grammes of powder and 900 grams of shot". The load for the 26mm gun is "100 grammes powder, 600 grammes of shot". Shot size 1, 0, or 00 are the recommended sizes.
Somebody was buying those guns.

Best,
Ted