Just some comments, after looking thru some files..
The List R-3948 is later than the Cobra time frame list numbers.
That list number does not appear in the Holley master list...but I'm thinking you knew that already.
The only S2CS p/n was on the '66 GT-350; all Cobra carbs were C5AE...
...and that GT-350 one was an R-3259
The 3-digit date code does place it as pre-1970