I can't positively ID whether it's for a GS.
The tailshaft housing casting date is very early, July 1964, might not be the GS version..... if there is even a difference.
Input shaft would have 3 grooves for the close-ratio 'high-nickel' GS trans. Tough to see what this one has.
It may be a 64-65 small block trans, wide ratio I believe.
The aluminum side covers were upgraded to cast iron sometime during 1965 since the alum ones cracked.... as this one did.
Others here know the small-block trans better than I do....