Tom,
I just perused my file of old Remington catalogs. My 1966 catalog indicates SS was standard only on 7mm Mag and .30-.338 Mag. Recommended on .22-250.
By 1970 SS was standard on all chamberings
EXCEPT: .222 Rem, .223 Rem, .222 Rem mag, .308 and .30-06.
My next newer catalog is a '74. It states that SS barrels are standard on all centerfire M-40X rifles... That's as close as I can pin it down... Hope that helps.
Grasshopper