Mark, "noire" means black in French. "Charge normale" should be the working load rather than the proof load, and it's 4.5 grams of black powder pushing 30 grams of shot (1 1/16 oz). Back in the late 19th century, that exact same load was the PROOF load for a standard 20 gauge, so I'd say your gun is pretty stout. T2 was an early bulk smokeless powder, and that's a proof load. The marks on the barrels (crown enclosed in branches) indicates a black powder proof, and the fact there are two on each barrel indicate a superior (higher level) proof. Your gun was therefore proofed with both black and bulk smokeless powder, which was common practice.
It's very difficult to date guns by French proofmarks. The crown over PT dates to 1900 according to my information. Some different marks came into effect in 1924, and they're absent on yours, so I'd put it in the 1900-1924 period.
Someone might have a Manufrance serial number table to give you more specific information on date of production.
