Without seeing the gun it is only possible to pass an opinion ,but looking at the pictures and from your text I make the following comments .
There are only one set of proof stamps on the action flats , as there are marks from different periods on the barrels this suggests that one set of barrels is from another gun,you also say the forend wont fit one set so this adds weight to the argument.
Yes, that is correct. The 12 gauge barrels are original, the 16 gauge barrels have a serial number indicating 1900 manufacture date. The kicker is that the barrels that fit the forend are the 16 gauge set.
