Look to see if there's a metal tag on one of the bolts that holds the cover on,if so it may have the ratio stamped on it.
If not here's the formula to figure it out. https://spicerparts.com/calculators/...tio-calculator

 

Using the second link Y2Keglide posted above shows the car originally came equipped with a 3:07 posi. I copied and posted the chart below.

Rear Axle Codes

COA 2.56:1 COM 2.56:1 COZ 3.73:1 Locking

COB 2.56:1 CON 2.56:1 Locking CRA 3.07:1

COC 2.73:1 COO 4.10:1 CRB 3.31:1

COD 2.73:1 Locking COP 2.73:1 Locking CRC 3.55:1

COE 3.08:1 COR 3.73:1 Locking CRD3.73:1

COF 3.08:1 Locking COS3.07:1 CRE4.10:1

COG3.36:1 COT3.07:1 Locking CRF2.56:1

COH3.36:1 Locking COU 3.31:1 CRG 2.56:1 Locking

COI 3.08:1 COV 3.31:1 Locking CRH 3.08:1

COJ 3.08:1 Locking COW 3.55:1 CRI 3.08:1 Locking

*** 3.36:1 *** 3.55:1 Locking CRX 2.73:1

COL 3.36:1 Locking COY 3.73:1 CRY 2.73:1 Locking

Rearend Code Example: COZ0327GE - (Chevy sourced 3.73 Posi, Assembled on March 27 w/ Eaton Posi)

 

Thanks. You saved an old man from having to jack up the car again and slide under the car again. ( It ain’t as easy as it once was).

 

awesome , my 81 has a GM corporate 8.5" 10 bolt. with some small parts of 12 bolt internals and hardened axles. since its street i'm opting for an aluminum housing. also a 40 gallon gas tank. looking to get 40 miles per gallon for 1600 miles per tank of fuel. Designed a 10 speed auto transmission.
Also, designing a mechanized fuel injection and induction system. making a very high torque upper band engine at very high volumetric efficiencies and mechanized timing that can intermittently alternate cylinder fuel and fire in a high rpm high torque engine. Big bore small block