The Hoyem book “The History and Development of Small Arms Ammunition”, Volume 3, British Sporting Rifle Cartridges mentions this cartridge on page 77.

He shows 3 examples of the cartridge, with express paper patch bullet as well as a longer NSW type lead bullet.

The edited Hoyem quote follows:
“These early cartridges have the .626” rim, .....(describes rolled brass case)......etc.
Bullet is probably the 270-grain express, charge unknown but likely 70 to 75 grains.
A military version with 480-grain bullet was loaded for the Alexander Henry falling-block single shot police carbine used in New South Wales, Australia during the 1870-1880 period. A .450-2 1/2” Boxer cartridge was used in one of the Westley Richards arms submitted for the 1866-1869 Breech Loading Rifle Trials.

Rare & Interesting for sure,
Mike