I have some info from Ned Roberts' later writing, which is very interesting. Don't feel like posting it tonight, but will tomorrow.
John, any follow on with this. I’m interested in the original source writing from Mr Roberts that got the spark lit on the 257R.
It's from Ned Robert's book
Big Game Hunting, published in 1947. He developed the cartridge in the 1920s by necking down the 7x57--but a more sloping shoulder, which back then was considered better than a sharper shoulder. When Remington adopted it as a factory round in the 1930s, they used the 7x57's shoulder angle.
Back then the slowest-burning powder available was probably IMR 4064. By the time Roberts published his book IMR 4350 had been around for seven years, and he decided to see what it would do in Remington's version of his round. Here's the quote from his book:
"We can hand-load the cartridge with 48 grains weight of DuPont I.M.R. #4350 powder and the 100-grain O.P. [open-point] bullet to give 3400 ft. sec. muzzle velocity... The cartridge hand-loaded with 47 grains of I.M.R. #4350 and 117-grin O.P. or S.P. [soft-point] bullet gives 3000 ft. sec. muzzle velocity; by using 45 grains weight I.M.R. #4350 powder and the 125 grains Barnes S.P. bullet we get 2,900 ft. dec, muzzle velocity."
Now, pressure-tested data from that era, such as that found in Phil Sharpe's book
Complete Guide to Handloading does not show charges anywhere near those for similar bullets, no doubt due to using the SAAMI pressure for the .257 Roberts, which is pretty mild. So obviously Ned Roberts was still doing a little "wildcatting' with his handloads. And Sharpe's data is very similar to today's non-+P data, so I doubt IMR 4350 changed significantly.
Years ago I pushed beyond the standard .257 Roberts data for IMR 4350 with couple of 100-grain bullets, working up to 47.5 grains for around 3250 from the 24" barrel of the Remington 722 inherited from my paternal grandmother. There were none of the traditional indications of high pressure, such as hard bolt lift, ejector-hole marks on the case head, etc.--but Sharpe reported 3400 fps from a very similar load. So I wonder about the barrel length of Roberts' rifle, which he doesn't mention.