It is not the 8.5" 10 bolt. It is the 9.375" 12 bolt, used from '55-'62. The AJ code may actually be a 3.90, rather than a 3.70. If you're checking from the same source I'm using, there is a typo on the '59 specs, it lists it as a 3.70. Same code for '58 is 3.90, and the shop manula makes no reference to a 3.70 gear. The only options were 3.90, 3.38, and 4.10. There was also a 3.92 gear, but that was the optional Spicer 45 Positraction axle, a one year only option on Chevrolet (was the standard equipment rear, with or without Posi, on GMC).
