It's a rough but matching and 100% functional 1873 French Ordnance revolver. The catch is that someone at some time opened the chambers a little bit so that a .45 Short Colt fits perfectly. When I saw that the chambers were reamed, my immediate 1st thought was "reamed for .45 ACP by the French underground (or a bubba) " but .45 ACP does not fit. .455 Webley also does not fit. I happened to have a box each of .45 Short Colt (a.k.a. 450 ) for an Adams I planned on buying but never did, and the rounds fit exactly and the cylinder turns perfectly. Of course I did not try to fire these rounds. I went online and tried to compare pressures and fps data between 450 and 11mm French and they seem similiar, but my Fiocchi 450 rounds are smokeless and I have read they load them hot. I did not want to experiment with ammo of unknown pressure. If I had Black Powder 450 I might think about it, as the 11mm's were of course BP rounds.
My guess is that when these revolvers were cheap back in the day, and no one could give away French or Italian revolvers someone got their hands on this 1873 and tried to alter it to fit a more available cartridge. It's a clean, well done reaming, seems like it was done with a power hand tool rather than a file or something. Whether it was meant for 450 or cut-down .45 Long Colt cases that were handloaded, I can't tell.
Any ideas? I want to be able to warn a buyer that the 450 may not be suitable for the gun, because of course I will be selling it cheap as an altered piece. Best bet, I think would be for a new owner to try to obtain a junk 1873 and swap the cylinder. It wouldn't be matching, but this piece is externally rough anyway, with a good shootable bore, but not very collectible.