What type is the rifling?
If the barrel was from a Martini, it will be Henry rifling. If that is what was done, I could see much of the breech - 2-3” being cut off to thread and chamber for a cartridge such as a 45-70. That would explain the lack of Knox form and the smaller breech diameter of the barrel as well as the lack of proofs.
The nitro stamp is on the action which makes sense for a 303 which it undoubtedly was.
What about the bolt face? Will the extractor fit over a 45-70 rim?
One of these chambered in 45-70 would be pretty fun indeed.