I think they would have been like the ones on the 1966 on the top of the picture. A 1979 is in the middle and a 1988 is at the bottom. Now is the darkness of the 1966 stocks due to oily hands, gun oil, or were they that way initially?
The pre 70 Series GCNM did not have a built up finish, and are normally stained a flat dark color. The 70 Series are finished in some type of clear coat, and are normally a more natural color with just a little shine.
The photos posted by dakota are a good illustration of this.