To determine if it is a 12 bolt there should be 12 3/8-20 holes on the back side of the gear. Count the number of teeth on the pinion there should be 8, and then the ring gear's teeth, there should be 39. You now need to put a caliper on the pinion shaft to see if it is 1.438" in diameter, or 1.625" in diameter. If it is the larger, congratulations you have a 12 bolt 4.88 gear set for a passenger car; and not a truck application.
There should be numbers stamped into the ring gear and etched on the end of the pinion. Only the stamped numbers are important in identifying who made it.
If the numbers read 69-0033-1 then you have a four series carrier Richmond Gear Co, product. If the numbers read 69-0308-1, it is also a Richmond gear but it is for a three series carrier.
A number stamped 01-888488 implies it is a Strange-US Gear product for a 4 series car twelve bolt.
GM12/488C or P indicates a Precision Gear Set.
GM part part number 3961192 4.88 gear 8.75" OD HD
I can not find a listing for Zoom Gear anymore as they were real popular in the sixties. But I'll keep looking.
Larger Dave
