hey jl,
welcome to Automotive Forums..
your brother is likely right. so, most GM rear ends
either had a tag near rear diff, or stamped with a 7-digit
alpha/numeric on the back side of the tube/housing..
first 2 are ratio, second set is month/yr, third is day, last single is a/b/c shift
since chevy was cranking out more than buick/olds/pontiac,
it is probably from a caprice, monte, or impala..
another possible way to figure it out (if no tag; usually removed, and
if you cannot find stamping), is to take pic of the 10-bolt rear end and post..
pics:
https://imgur.com/a/RN2XIBJ
hope this helps..