Any rear end that old may well have had the gears changed at some point. The ratio may be the same as original or some other ratio. You would need to count gear teeth, dividing ring and pinion count to determine ratio, or look for stamping on the od of the ring gear to verify ratio.

You can get a rough idea by rotating the tire off the ground and counting driveshaft revolutions per single complete revolution of the tire.

The BOM number stamped on the housing will give you information from the date of manufacture, etc.