For '67-'70 UCA the number of bolts on the ball joints is the only difference. Both type are dimensionally the same. It's been said that the change over was mid-year in '69. Prior to that all the UCA had 4 bolt ball joints, after that they all had 3 bolt. In reality the assembly line used which ever type they had on hand. Apparently the supply of UCA changed over gradually according to what ever the supplier provided to Ford and during the change over period both types of UCA were available on the assembly line so some cars ended up with 1 of each. I've heard of this happening on some '68 models as well as '69 and '70 models so the new 3 bolt UCA had to be available while the '68 models were still in production and 4 bolt supplies lasted into the '70 production run. I actually bought a used original set from a '69 so I could use the 4 bolt wedge kit I had and it turned out be 1 of each type of ball joint.
Not many people are aware of this but the '71-'73 UCA are actually just a little longer than the earlier UCA. They are all 3 bolt and completely interchangable with the earlier UCA so rather than tool up for such a minor change most suppliers offer just 1 UCA to cover all the '67-'73 models. Just thought I'd add that little bit of trivia to finish of the differences you asked about.