The Celje Regional Museum came up with a high res picture, and some more info, of this bad boy.
total length 910 mm,
length of gun barrel 458 mm,
caliber 9,8 mm
I suspect that the loading method was by unscrewing the barrel and carefully fitting a ball into the breech, which would explain the large flat on the barrel.
With a relatively small .38 caliber, carefully fitted and patched ball in the breech, I think this guy would have excellent performance potential.
Just to add here what you said on the airgunbbs: "It is marked "Cortina d'Ampezzo" which is the hometown of Bartholomew Girardoni."