In a word, no.
Apologies if this comes across as offensive - but what makes you think that it is a) an Austrian M1854 "Lorenz" rifle? b) that it has anything whatsoever to do with the Civil War?
I think it is a "home-brew" built around a carved-up original stock and trigger guard from a Lorenz M/54 "Jaeger". Considering what is obviously
wrong for a Lorenz - lock (unidentifiable), hammer (
French
style), nipple bolster (village blacksmith), backsight.... someone tell me what's
right!
The Lorenz had a 13.9mm (.547) bore. Old ex-service BP rifles were often reamed out to make smoothbores for use as shotguns. In which case, one would expect a smooth-bore of around 0.6". Never 0.75 - that would be an enormous waste of effort, and in most cases the original barrel would not be thick enough to permit reaming to 12-gauge. This was probably always a 12-gauge shotgun barrel - it is indeed marked with a 12.
And the XXXX \ / III marking is typical cold-chisel marking for a set of components, to stop sets getting mixed up, in 19th century small-scale (hand) batch production. These were placed where they would not be visible on the finished gun, and were nothing to do with regiments etc, just in case anybody's thinking that!