Welcome to the forum.
Is the gun on face, i.e. are the breech ends of both barrels in contact with the breech face? Many times when a double is fired, once, it is the right barrel which is shot. Over many years of this the recoil effects on the right side will wear the hook/pin in such a way as to skew the barrels like this. Even though it was an inexpensive gun when made, the alignment was most likely much better than it is now.
Short story, I'd look into that as a way to correct the alignment issue rather than filing the receiver, which would have to be annealed before you could do so anyway.
Good luck, SRH
