Awhile ago I set this display up (see pictures below) so you can see the difference between the two styles (I have since learned there were three, more on that later) of upper control arms

The three styles can be differentiated by the basic differences in the pivot shaft:

           One style had a bushing that screwed onto the pivot shaft (see the detail pictures of the pivot shafts below)
           One style had a bushing retaining Nut.
           One style had a bushing retaining Bolt (it appears there two different style shafts that had the bolt).

Under this set of pictures is another set that deals with the Pivot Shaft differences

On the left side of the pictures are the left upper control arms with the bolt style retainer. On the right side of the pictures are the right upper control arms of the nut style retainer.

As a general rule bolt style retainer upper control arms were on 1964 and 1968 GM A-Body cars. The bushing shaft on these cars had bolt holes in the control arm shaft that small bolts threaded into to hold the large bushing-retaining washer in place.

As a general rule nut style retainer upper control arms were on 1969 thru 1972 GM A-Body cars. The bushing shaft on these cars had threaded ends and a large nut that screwed directly onto the shaft to hold the large bushing-retaining washer in place.

All that being said, I have both styles in my shop and they are the same, they use the same bushings for both stlyies. Also I have found the two styles mixed up on the different era of cars. I.E. the 69 thru 72 style on 66 & 68, and 64 & 67 style on 69 thru 72 cars. There could have been two different vendors supplying them back then. GM tried to keep as many parts the same between all the A-bodies as possible.

The important things to note 1) There are no difference in the arms themselves 2) Both styles are interchangeable on these cars and arms. 3) Some people refer to the nut style retainer as a control arm from a car equipped with heave duty suspension (this is not true). 4) In my opinion the nut style retainer is stronger then the bolt, however this style seems to have more problems with rust in the threads of the control arm shaft.