Info I have shows the easiest way to tell a 69-70 from earlier is the top has a "cross" indentation in the top on a 69-70, and earlier ones don't. Just A "U" shape. Some 70's supposedly were pinch-weld type.
A 66 (and late 65) had a square bar end where the threaded rod travelled thru the open end. 70 had a rounder one.