Having considered your dilemma a bit further, I think that you will have to take a series of internal diameter measurements of the chamber and the barrel to try and unravel the conundrum.
W Ford the gunmaker was a renowned barrel borer and known for making "specials". Around 1880 he advocated reworking of chamber, cone and barrel forward of the normal position of the cone in converting paper-case chambered guns to brass-case guns - a rather controversial method, considered by others as likely to render the gun "out of proof".
By doing the measurements you may get some insight as to whether the gun was originally built as a brass-case ("chamberless") gun or if it is a converted paper-case gun { assuming that the chamber and bore have not been "modified/cleaned-up/altered" at a later date, thereby only adding to the ??? history}.
