Bill,
In glancing in my '71 NEC, the service conductors could have been as large as 1/0 Cu or 3/0 Al (175 Amps by the Notes to the Ampacity Tables) and the GEC only had to be #4 Al or #6 Cu.
I wonder if that Phillips head screw at the top-right of the neutral assembly is going through to the box? The bonding screws, when used, weren't green back then.
The box looks like ITE. Do you know? The breakers are an assortment of manufacturers, to my eyes.
Al