Backsight is for a No. 5.
It would be interesting to dismantle the rifle to see how it was done.
With a lathe, it is no big deal to sleeve a salvaged muzzle section onto a barrel to get the front sight and bayonet lugs. Cut the barrel, bore out the cutoff muzzle, turn down new muzzle area. Could be sweated in place, or Loctite sleeve and bearing bonding agent used. A careful inspection of the muzzle would be in order.