Here are the "inside" views.

[Linked Image from i.imgur.com]

I see from the Birmingham mark, that it was proofed in 1963, a couple of years after the history I read said they closed.

When assembling the gun, the forend needs to start out at about 45 degrees to the bore, then pressed down , lowering [raising] the cocking dogs, to then latch on the Anson push rod.