Although you really can't see the first letter (before the "S"), more than likely it's "BS" which was the standard 3.31 rear axle for your SS 350. The "E" is for an Eaton Posi, optional for your car. The "G1" is the first shift of the Detroit Gear and Axle Plant.
Look closer at the area in front of the "S" and you might be able to see a faint "B".
Ed