The 450 No.1 Express is the 2-3/4” version of the 500/450 family.
Think it was originated by Westley Richards.
Agree that a chamber cast and even slugging or otherwise measuring the bore will help in selecting a good bullet for it.
I have a couple No.1 Express rifles and they so far have done just fine with .458” cast bullets.
Also, I have noticed that the 450 sporting rifles tend to have more consistent bores down around true .450” while military rifles are all over the place as far as bore measurements.
The No.1 Express is purely a sporting cartridge though for that matter.

I really wanted this rifle to be buddies with my Purdey 450 but I do need to exercise some restraint occasionally.