Hey Argo,

The chunk of metal that splits the "Nitro ... Proof" in half in the flat photo, looks just like the cocking lugs on my Beesley built Lancaster self-opener (ca 1894). They were known as the "Wrist Breakers".
With the sidelever and ejectors, I think the gun uses the Beesley patent, cock on close, action.
This is the action Beesley sold to Purdey probably not long after this gun was built.
The gun is cool.

Toby or Lagopus can hopefully add more.

All Best,

Joe in Charlotte