Daryl:
Its my understanding that the words "Nitro Proof" were used from 1896 onwards, anything before that not nitro proof. That is the London Proof House, Birmingham a bit different.
The actual NP proof mark including Knight's arm with scimitar was added from 1904 onwards I believe.

Guess I better check my chamber length!
All those offerings from Rebel are 2 3/4 by the looks.

I don't have a "proper" rook rifle and that's an itch I will need to scratch. I have been using a W.P.Jones side-lever hammer single for rook rifle comps but it has original chambering and proof for the .22 Rimfire Long, black powder load.
It shoots the .22 CB Long cartridges to the sights, but is frowned upon under BGR rules.
Martini-actioned rook rifles are a sensible standby, and we allow .310 Cadets for our monthly range-day shoots, but not pukka for serious competition.

One day...

Of course, if we actually had rooks...
...then a 28-bore shotgun like this one would probably be hard to beat!