Re: Trying to identify year, make and model.
The obvious step is to try to identify any outlaw (or lawman) with the initials "C.A.". From what little I know about Australian history, I don't think a law officer would have 1) had a Spanish(?) copy of an English revolver, 2) would have carried his own personal weapon 3) would have carved up a government issue weapon that way and 4) have killed that many men, assuming that is what the notches mean.
So I am inclined toward the "bushranger" idea; that gun would probably date to the 1870's; the period when many percussion revolvers were being converted to cartridges. Of course, it could have belonged to anyone in a rough era, even to a someone who was not a recognized "bushranger", but just a plain old unromantic criminal.