There were NO stainless Colt 1911 series pistols made then. Colt didn't offer stainless Government Models until the mid 1980's.
The 70CS serial number indicates a Colt factory satin nickel finish.
The same gun in blue had a 70BS serial number.
This finish was an earlier satin finish then the later "Coltguard" finish. Colt said the finish on the 70CS was a satin nickel, but for some reason they used "SC" in the serial number, often taken to mean Satin Chrome.
A good way to tell stainless from bright or satin nickel is to compare them to something you know is stainless steel. A kitchen utensil will do.
Stainless steel has a "white" tint to the metal, nickel has a distinct "yellow" or amber tint that's very noticeable when compared to stainless.
Hard Chrome has a blue tint.