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 . I would also surmise that it has been re hardened at some point Westleys forend are normally blacked. Agree that the marks on the barrel were probably caused by a hand guard . You say the chambers are 3" on both sets ? This would be most unusual on a gun of its type and vintage and extremely rare on a 16 bore and as it is marked for 1oz then they have certainly been deepened from 2&1/2" as I suspect the 12 Bore was as well .
Please have the barrel checked most carefully be fore you attempt to us this gun . I for one certainly hope that every is OK and that it can be made to function correctly as it appears to be a nice enough gun .
