Was able to land a really nice PSA 9
1972 Topps #607 Frank Duffy 'yellow letter' variation (card on the right), and realized there seems to generally be three, not two, versions of the card.
The one on left is the full green version, but between it and the 'fully' yellow version, there is a bit of a hybrid example, where the letter bottom green is lighter, but intact, and the upper green portions on the inside of the letters are almost completely gone...
1972duffy607greenyellowvarprogression.jpg
I assume this is due to a 'weakening' of the cyan ink, because you can see how
(most notably) the sky moves from dark blue to much lighter blue across the three versions.