I believe it could be refering to the early Rem actions that didn't have a letter in the first position of the serial "number".
The first Rem 700 were built in the early 60's. At that time there was no letter in the serial number (the lowest I've seen is a 4 digit number), The numbers only continued for a fair while and then the "A" series began, they're at "E" now I believe.
I've seem some actions that had no number on the outside of the action, the number was recorded on the inside left of the action.
The early actions are sought after by many folks (I prefer them too) for custom rebuilds. Another indicator of an older action is whether or not it has the 2 rear sight block screws on the left side of the action. Early models had "stripper clip" slots in the action and the safety "knob" was a larger "paddle" type rather than the rounded knob. Some early models also lacked the bolt rail slot in the bolt and looked very much the same as the 722 and 721 bolts.
I'd imagine that it'd bring about $250.00 + or - $25.00 (a short action might bring a little more as they seem to be a little more difficult to get).
I usually replace the floorplate with a Dakota model or perhaps a Badger Ordnance. The Rem trigger is good and I use them, if it's missing from the one you're examining a replacement is between $25 and $70 depending on where you pick it up.