I have a 1969 chevy el camio that came stock with a dana 4/11 positrac rearend. how can this be so?

this car was my grandfathers, I inherited it ,I have alot of the orig. paper work , orig. purches order pink, slip and so on . the orig. purches order says he ordered a positrac rearend but dosn't say what kind. when i started working on the car and got to the rearend i had to purches parts for a dana 4/11 positrac rearend from a 1967 oldsmobile. now i've been around this car all my life and the rearend has never been changed . so i come with the question how does a 1969 chevy el camino come stock with a dana 4/11 positrac rearend from a 1967 oldsmobile and how do i prove it when general motors refers me to their heritage center and they tell me their arcives only go back to 1977?