I recently was at the auto swap meet in Pomona. I was going to buy a P/S gearbox that the seller indentified as 68-72 Chevelle. However I noticed that the rag coupler shaft was a smaller diameter than what I had. The rest of the gearbox looked the same with the same casting number on the housing. I didn't buy it. Any ideas of what that gearbox was from?

The Saginaw recirculating ball, power steering gear housing can accept a very large number of components that extend from today's pickup trucks all the way back to the mid 1960s. So you could have a very old casting with modern "guts." Part of it being a 3/4 diameter input shaft rather than the older 60s to mid 70s 13/16 diameter input shaft.
This paper will give you a lot of the information. It is geared toward placing modern fast ratio gears in early muscle cars but it contains a lot of information that concerns your question.
http://www.corvettefaq.com/acar/A-CarFastRatioGear10JN2002.doc