Apparently Southbend is totally clueless as to the origin of the .257 Roberts cartridge. It was developed by a schoolteacher in upper New York State named N.H. (Ned) Roberts with assistance from other notable cartridge experimenters of his day, notably Col. Townsend Whelen and A.O. Neidner, along with suggestions by Harvey Donaldson of "Yours Truly, Harvey Donaldson" fame in "The
Handloader" magazine and who developed the .219 Donaldson Wasp of early benchrest fame. Ned Roberts, who, by the way, always called the cartridge he developed the ".25 Roberts", was already a shooting authority of note, being a collector of and an expert on caplock rifles and having written books about that subject. Roberts, a major in the "War To End All Wars" also wrote "Big Game Hunting" published after WW II.
Regarding commercial rifles that were chambered for this cartridge, Remington was chambering rifles for the .257 Remington Roberts before the Winchester M70 was even announced in the "American Rifleman" in late 1936. Winchester's pre-M70 Winchester turnbolt sporting rifle, the M54, started chambering for the now legitimized former wildcat .257 Remington Roberts in 1936. Remington had been chambering their M30 rifle, a much modified action based on the wartime M1917 Enfield action, for several cartridges but in 1934 the original M30 started to be chambered for several new cartridges including the .257 Remington Roberts.
Cartridges developed by individuals are considered wildcat cartridges until, and if, they are adopted for commercial chambering, at which time they are considered legitimized. There are several current legitimized cartridges now available, notably the .22-250 Remington (Remington's version of the wildcat .22-250 or "Gebby Varminter" as it was known when it was patented by gunsmith Jerry Gebby; the Remington .25-06 (Remington's version of the Neidner Arms wildcat .25 Neidner, and the Winchester .243 Winchester (Winchester's legitimization of the .243 Page Pooper, developed by noted gunwriter Warren Page). Major Roberts developed the .25 Roberts in the late 'Twenties and it was a wildcat until modified and legitimized by Remington in 1934 as the .257 Remington Roberts.