That looks like the same rear end that I have, it's an Eaton 1350 with vacuum shift.
You have the bracket for the vacuum pod, but I see the pod is missing.
Her's mine http://s675.photobucket.com/albums/vv111/JeffL_album/?action=view&current=100_0857.jpg
My truck is a 45 GMC, mine is original to the truck. It is possible this could be your original, Im not sure if these were available on a Chevy or not.

Last edited by JeffL; 10/07/2012 9:31 AM.