2 speed rear end in a '66

[Panelrodder]

Hey all. I've bothered you with stupid questions in the past but this one I feel is actually worth your time. I bought a 66 C10 a couple weeks, over month now actually, ago with the hopes of getting another one, hopefully in the next week, that I could use to build one good truck out of. The 66 C10 the I am working on getting my hands on has a straight six matted to a four speed tranny and finished off with a 2 speed rear end. My question is, is the two speed rear end factory or not. If not what would that rear end most likely have come from were it could still fit the width of the truck? Second, do any of you have a some links to information about these rearends. I'd love to understand they dynamics of them and also find out what ratios it has.
The PanelRodder