Rear end is a 12 bolt, code is BT032502, with an E underneath – which IDs it as a 3.55 Eaton posi unit from March 25, which seems to jive with the VIN date. I don’t know what the “02” at the end stands for, although the zero could have been a 6 or a G, very hard to read.
Probably a G2 after the date.
G=Detroit Gear and Axle
Like Kurt said the axle look good but no way to prove it is a real SS. My take is that unless someone went through all of the trouble to change out to multileaf rear springs, heater box, and radiator and not just the engine and trans, so I have to agree with KurtS there is a good probability it is a real SS, but no way to prove it.