Hello lancaster,
Thanks for the reply,

I have been able to contact the original owner of the rifle. He said he intended to go to Africa to hunt with the rifle, but political conflicts in Rhodesia at the time prevented his going. He told me the rifle was custom built for him / by him. He said he was actually an employee of Fred Wells at the time and did the stock work himself while so employed. The stock wood was special ordered "Red Cocobolo". He also said there was a 400 Wells Express caliber at the time and that it used .411 bullets. He liked the caliber but said he used "411" in the caliber name on his rifle so as to be sure it was identified as a 411 caliber, and not confused as .400 caliber. He also said he sent a fire-formed case to RCBS to have dies custom made, and that RCBS marked the dies 405-378 ... because the 405 Winchester cartridge used a .411 dia bullet. So I guess, this is in fact a "one of a kind cartridge". https://en.wikipedia.org/wiki/.405_Winchester