'72 Topps w/ Yellow Back?

[steve B]

64 Topps should be a single pass.

Into the early 1980's, many things were printed with a sort of modified 4 color process, where some special colors were printed directly from hand mixed ink. One of the expensive parts of printing then was the color separation.
The card backs wouldn't have needed that, so they would have just been photographed, the plates made, and the colors mixed by the press operator.
There was a recipe for each color, so much of each thing, with bits of other colors to fine tune it.
For 64 Topps, there was probably a red, altered with white and maybe a touch of yellow and black to make the usual color.
It was possible to mix the color wrong, making it too light, or too dark, etc. But the various examples seem a bit more extreme than would have been acceptable as "about right"
There wouldn't be any reason to do two identical plates and keep the perfectly registered. It would make the cost nearly double, but you could save some by using the same mask to make the plates.
Knowing Topps, I can't see them keeping two things registered for a day or tww, let alone several years.

From the scans I'm not seeing any signs that would make me think it was a chemical change, like something spilled that bleached it a bit. Those things usually raise the paper fibers.

If the ink was made with colorants that weren't lightfast, it's possible red could fade but not the yellow that would basically be in the same ink.
I'm not sure I have an extra 72 or 64 I could leave out in a sunny spot for a while. I know I have extra 91s...