As I understand it, the cartridge was actually introduced by Purdeys as a "nitro for black" loading of previous BP 40-cal cartridges of varying lengths, settling on 3 inches as a standard moving forward.
Obviously something more powerful might have stood the test of time a little better than the .400 Purdey did!

Nevertheless, I too have a good stash of original brass tucked away for the day I find an affordable rifle so-chambered!
(note pigs flying overhead...)