Here's another insert set from the '60s that I recently knocked off, the 1969 decal set. Topps put out 48 decals in the set, two players from each team. The two Cubs were
and Ron Santo.
As you can see, the decals were almost identical in design to the '69 base set. There were a couple differences, though. First, the decals were much smaller, 1 3/4" x 2 1/8" Second, the color of the inside of the circle changed. In the base set, the Cubs were all orange. With these, you see that two different colors were used.
The final difference may be the most noticable: different pictures. For most of the players, including the two Cubs, Topps used a picture that was different from the base set picture.
Here are the two Santos side by side.
For Fergie, different need to be "different" because though Topps didn't use the same picture with the two from '69
the decal is the same picture that they used on the '68 card.