Hector et al

Is it possible this is a shotgun first with a set of later Rifle Barrels? It would answer the questions about when the original gun was made with the Rifle Barels made in the period shortly before 1904. It seems odd to me to make a rifle on the 12 gauge frame. A 20 or even a 16 but the 12 is very large as you can tell.

If it is I might be hesitant about a Nitro Proof. I would make sure a knowledgable person viewed the action and barrels before going ahead with it.