I have found something that may not be just a 66 feature, maybe just early Broncos, I took pics and will make a post about it.
The manual steering box on my 66 and a friends both have just a round portion of the input shaft for the bolt on the coupling, it is rounded or detented all the way around the input shaft. My friends slipped on her one day when trying to turn. On mine, it was on the trailer, we tried to turn a little and the wheel hit the trailer frame, and we had it slip. That wasn't something I liked, so I pulled it and checked, it was the same way.
My later 71 box has a rounded portion for the bolt, but its only at one spot, sorta keying the coupling bolt and helping prevent slipping. So I am in the middle of changing them out right now as I feel safer that way.