[Captainfab]

If you want to know what gears are in the differential and whether it is a posi or open diff, just pull the cover and have a look. There's no telling what could have been changed in the past 50 years. More than likely the oil needs changed anyway since a differential is one of the most neglected parts of a vehicle. It'll only cost you a couple quarts of gear oil, a cover gasket and a little time. Then you will know exactly what you have and the peace of mind knowing that your differential has fresh oil.