If the Perches on the axle tubes are angled where the Control arms bolt on, then you have a 60's rearend. On the back of the center housing, usually off to the side of the Differential Cover, it should have a casting date code on it. A letter for the month, and then 3 numbers. The last number will be the year (ex "H123", the 3 would stand for 1963). If your dealing with a local Parts Store, it seems how they reference rear ends and tranny's doesn't seem to match what is the common lingo here. This is what I found anyway. In the Summit Performance Parts catalog, and in the LMC Truck parts catalog, they show you the bolt pattern and gasket for the different rear ends. If it's a 2 wheel drive, your mainly dealing with a 10 bolt or 12 bolt truck rearend. The 10 bolt has 10 bolts and a perfectly round bolt pattern on the Rear Diff Cover. The 12 bolt has 12 bolts and is an oblong pattern. Find the year from the casting, have the Parts Store look it up until they have the correct gasket pattern, and this should get you in the ball park for which reaer end they need to look at for the seal. As I mentioned, out here anyway, the part stores have no reference to 10 bolt, 12 bolt etc. If possible, it would be a good idea to take the old seal along. Hopefully this will get you started anyway.
