"Poudre Pyroxylee" on the barrel flats means Nitro Powder.
"65" = 65mm chamber = 2-9/16" LOW-POWERED shells.
The gun was most likely made after 1925, when ammo companies introduced more powerful shotshells loaded with different powders to higher pressures, lengthening most shells & chambers to suit.
The changes of 1925 necessitated marking the long and short chambered barrels differently - before that date, since there WAS no longer ammo, there was no need.
Gun barrels chambered/proved, prior to 1924, would have been marked simply with a "12", "16", etc, maybe enclosed within a diamond or circle with the letter "c".
After 1924, though, gun barrels were required to be marked with the exact length - usually in MM for Euro guns - as in: 16/65, 16/70 (2-3/4"), 12/65, 12/70 & 12/76 (3").
IF a qualified double gunsmith (NOT an 870 parts-changer) OK's it as safe to fire, special, low-powered ammo is available from the likes of RST, PolyWad & others.
As posted above do NOT shoot ANY modern ammo through it - even so-called "low brass" or target loads.
The pressure curve of modern ammo is different, and the almost 100 y.o. metal of that gun is not of the same alloys as today's stronger steels.
It's most likely worth about a max of $200, +/-, since whomever made it wasn't proud enough to put their name on either barrels or lockplate(s).
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