Is it possible that it was once a damascus steel barrel( or anything else) that has been relined with a set of steel tubes? It is common to cut the tubes forward of the chambers, machine down the tubes to line the chamber, but leave the remaining length of the barrels outside diameter. TYhe short section of the barrel which is the chamber can then be bored out to accept the sleeves. That allows the use of the same breech plugs, so you don't have to rework the stock to fit a new tang, etc.
Without actually seeing the gun, its hard to tell, but I know of a breechloading L.C.Smith that was restored this way, because the artist who did the restoration did not want to have to tackle making and welding on the underbarrel lugs for the pivot, or have to machine and replace the existing extractors. The gun is an heirloom, and the owner was willing to spend the extra money to replace the barrels to keep the gun a shooter so he could pass it on to his sons. That is the only reason I can imagine why the barrels would be made of what appear to be different metals. It also might explain why proof marks are not evident on the barrel's undersides- they went with the cut off sections of the barrels.
