Rock Auto shows pictures of all the 56 U joints they sell, and they all show inner cir-clips on all 4 caps, on every version..............
It shows the dimension as 2.256" as the distance from outer edge of the clips.
The one you linked to is a double cardan CV type joint from my experience. I also owned a 56 probably close to 50 years ago, so as Jack said, too long ago to remember that particular detail.