The actual back is shown in the photo below (of Yankee Stadium). There are six columns (A thru F) and five rows -- thirty options for the sticker backs.

A1 B1 C1 D1 E1 F1
A2 B2 C2 D2 E2 F2
A3 B3 C3 D3 E3 F3
A4 B4 C4 D4 E4 F4
A5 B5 C5 D5 E5 F5

Upper left corner is A1, lower left corner is A5, upper right corner is F1, and lower right corner is F5.

I've created a graphical representation of the puzzle pieces.
https://pasteboard.co/H3lpwxK.png

Back puzzle complete in 30 pieces. There are variations where the same front stickers will have different back puzzle pieces. While not numbered, I have designated them by row (letters) and numbers (columns), starting in the upper left corner. Thus the upper left corner puzzle piece will be designated A1, while the lower right corner will be F5.