[Fleerfan]

I do not believe there are back variations for complete panels.

Individual cards can be found with different backs, but that is because each player is on multiple panels.

Since players appear on multiple panels, you can find the individual Alan Trammell card with different backs because on the Cooper/Wilson/Trammell panel the Trammell back is the ball-strike indicator, but on the Jackson/Bell/Trammell panel, the Trammell back is the locker offer.

A master set is 144 panels. I put together the master set through multiple lots, and never came across any back variations.

Looking at the pictures of uncut sheets, there are 72 National League panels and 72 American League panels, which equals the 144 master set. Therefore, no panel combination would have been double printed, so it does not seem possible that there would be 2 panels with the same combination of players on the front that would have different backs.