Tom,
NEF states that receivers made before a specified serial number are NOT suitable for fitting with rifle barrels.
I don't recall the s/n breakpoint, but suspect that it has to do with a change in materials and/or manufacturing methods. a change from cast iron to investment cast alloy steel for example. As we know, Ruger made it's great reputation on investment cast steel receivers and major parts for almost all of it's product line (bolt action and single-shot centerfire, shotguns, revolvers, pistols, etc).
HTH
John