Might want to recheck your facts. The .257 Roberts started out in the 1920's as a wildcat of the 7x57 Mauser developed by Ned Roberts. Remington domesticated it in 1934.

The .243 Winchester was introduced in 1955 in the Winchester Model 70, based on the .308 Win which had itself only been introduced 3 years prior in 1952.

So the .257 Roberts has 21 years on the .243 Win, and that's just counting the years since it was standardized as a commercial cartridge. If you count it's wildcat days, it's got the .243 by another 10.

One thing to remember too about the current .257 Roberts factory ammo is that it's typicall loaded to the much lower pressures than modern rifles can handle to be safe in the old and much weaker Model 93 Spanish Mausers that were commonly rebarrelled in .257 Roberts back in the day. New production rifles chambered in .257 Roberts can handle higher (+P) pressures which allow the .257 Roberts to sneak up pretty close to the .25-06. But you'd have to handload to take advantage of this.

Mike