The earlier trucks mostly had the 10 bolt, which has a Spherical round rear cover on it. The 12 bolt (not the same as a car 12 bolt) will have 12 bolts in the rear cover, sort of oblong shaped, with a vertical hump that sticks out for ring gear clearence. Looking at the the cover from the back of the vehicle, to the right side of the cover, there should be some alpha/numeric numbers cast into the housing. On the 66 they are down inside a recess on the housing. this will tell you what year it is ex: C126 would be March 12,1966. If you pull the cover, on the edge of the ring gear, you will find a part number and 2 more numbers, ex: 41 and 11. These are the number of teeth on the ring and pinion gear. Devide the big by the small and that will tell you the gear ratio
