This is the original rimmed (flanged) .350 Rigby introduced in 1899 and later known as the .400/.350 Nitro Express. Used in double and single shot rifles and also in bolt actions. The long actions with a magazine designed for the rimmed case were built specially - and only - for John Rigby by Mauser in Oberndorf. The cartridge is not to be confused with the later .350 Rigby Magnum and it's rimmed version, the .350 No.2, which were loaded with bullets of 225 grains weight. The flanged .350 No.2 used the same case as the older .400/.350 NE but with the lighter bullet. Original ballistics were a solid or soft-nose bullet of 310 grains at a velocity of either 2000 fps or 2150 fps (depending on the source of the info and probably the ambient temperature at which it was measured). John 'Pondoro' Taylor was a great admirer of this cartridge. Brass is available from Bertram in Australia and bullets to the original design from Woodleigh, also in Australia. 

This text is based on information from “Cartridges of the World”, Hodgdon reloading manual, the cartridge designer and/or own resources.