Okay, if it were FACTORY it would be priceless or at least very spendy or valuable predicated on selling or buying.
That being said, there are NO factory 257 Roberts in the "AB" configuration. If the seller is telling you differently ask for PROOF, like a personal letter from William Ruger Sr. and get the serial number and call the factory records department.
The "AB" was first offered in the non prefix rifles 1967-1969. The "Standard Light Sporter" was offered on any caliber that would go down the skinny barrel. SO there are numerous calibers of non prefix "AB"s. The 257 Roberts was NOT offered in this time frame.
To the best of my knowledge there is only one non prefix #1 in 257 Roberts and that is a standard "B" rifle delivered in 1972 as FOB rifle.
The 257 Roberts was first cataloged in 1980. AT that time it was cataloged as a "B" and has never been cataloged as anything other than a "B" with a 26" barrel from then till discontinued in 2003.
The only exception was the run of stainless/walnut "A" Light Sporters offered by Lipskys as a Dealer Special.
The AB was once again offered/cataloged in 1980-1983. At that time it was NOT offered in 257 Roberts. It was offered as a 223, 270, 7x57 and 30-06. More recently it was offered in the 204 Ruger.
There is another current post of a 22" 257 Roberts and the poster says he called the factory and the rifle is in the records as a "B" with a 26" barrel.
My very best educated guess is that it is a cut down factrory rifle. To check if you want, the barrel end of a true "A" barrel is .562" A "B" or 26" barrel cut off at the 22" point will mike right around .681.
Unless it can be PROVEN to be a factory "AB" which I seriously doudt, it's a $500 shooter/parts rifle. It can be sent back to the factory and re-barreled to the correct "B" 26" barrel for $396.40 plus shipping both ways.
That would result in a correct but somewhat spendy 257 "B" at $1000 or so when a guy was done.
Ross