This what Keith Luckhurst wrote back in an article 'Reloading the 404 Jeffery';

"My .404 started out life in 1928 as an original ‘Type A’ Oberndorf Mauser in 10.75x68mm. It was used to shoot an elephant in Tuli in about 1973. The shot was a frontal brain shot, which knocked down the animal, but it immediately got up and made off to Botswana with great enthusiasm. It was then decided that the conversion to 10.75x73mm, also known as the .404 Jeffrey, was in order."

My own 404 also started life as a Mauser Type A 10.75x68 suitably converted to the 404 Jeffery cartridge, presumably in Harare, Rhodesia where it was purchased and brought back to NZ.

Back in the late 70's when I got the rifle 404 ammo and components were hard enough to find let alone trying to find 10.75x68 stuff so I suppose after Kynoch closed down and RWS also limited its production of NE cartridges, converting 10.75x68 chambered rifles to 404 Jeffery was likely more an expedient for supply rather than anything else. Notwithstanding this I would much prefer the big 404 cartridge for large and dangerous game rather than the somewhat smaller 10.75 Mauser.