It's a .475 3 1/4" Nitro Express. This was Cogswell & Harrison's proprietary .475 bore introduced in 1907 as a .450 replacement for India. Holland's .500/.465 (actually originally referred to as "the India"), Eley's .475 No. 2, Jeffery's .475 No. 2, and Westley Richard's .476, also all retained as proprietary cartridges, were introduced at the same time for the same reason. The .470 had been introduced much earlier and had not been retained as proprietary, which is the only reason it became the most numerous of the .475 bores.

Since C & H retained the .475 3 1/4" as proprietary, most double rifles chambered for it were either theirs or Rodda's, who C & H licensed the use of the cartridge to.

The range of .450 replacements introduced in 1907 had nothing to do with events in the Sudan, as that market was far too tiny to have had that result. India dwarfed all other rifle markets for the British gun trade. The .475s were introduced to replace the .450s in India due to the tangle of enforcement errors that eventually ensued from the .303, .577/.450 Martini-Henry, and .577 Snider ban there in 1899. Contrary to popular belief, there was never an order banning the .450 Nitros in India.