That looks like one of the old differential mounted overdrive units. This one seems like it comes off of a HD truck, judging by the size. I used to have a '65 Galaxie ex-cop car that had a vacuum controlled OD mounted to the 9" that was much smaller, something like this:
http://www.stangerssite.com/honeodrive.html
The one on the Galaxie was fun..could engage from any of the gears for a 20% overdrive, so you theoretically had an 8 speed toploader, just remember to clutch before engaging or disengaging the OD.