Have a look in Truesdell's comprehensive survey entitled 'The Rifle and its Development for Hunting'. You will find precisely no references to the .400/.350, a single hunter (Frank Melland) who used a Rigby-built .350 double (almost certainly a .350 No.2), and five hunters who used the .350 Rigby Magnum (Wm Rainsford, Leslie Tarlton, Alfred Pease, Hesketh Prichard, and J. Stevenson Hamilton).

Admittedly, there are holes in Truesdell's work, and he was restricted to those who wrote about their firearms (many hunters didn't even mention calibre), but it provides a good overview IMO.