Over the years the tag often gets removed,,, AND LOST.

The 10- bolt rear will have 10 bolts on the cover and the 12-bolt will have 12 bolts holding the cover on.
This is a half ton pick-up I take it??

There should also be stamped numbers on the axle tube to one side or the other on the forward side,,, use a wire brush to look around the numbers are there.
http://www.nastyz28.com/camaro/zsusp.html#rearaxle

There is enough good in the worst of us and enough bad in the best of us that it does not behoove any of us to criticize the rest of us.
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Be yourself. If you are ever lost, It will be much easier to find yourself if you know who you are!