What are the parts numbers? They would give you a close approximation of their production dates.
To elaborate a bit, if I remember correctly, 1957 was the first year for 8 3/4 rear axles. In 59, the standard V8 rear axle ratio with Torqueflite was 2.93, with Powerflite (2 speed), it was 3.31 (to help compensate for the 1.72 low gear ratio of the Powerflite vs. the 2.45 low gear ratio of the Torqueflite).
There was also an 8 1/4 rear in this era that externally looked just like an 8 3/4, but the ring gear was smaller. Mostly for sixes I think. Don't remember what years this 8 1/4 was offered.
