The AP lenses with FPL53 did not use ZKN7. Because of issues with glass quality, AP had stopped using ZKN7 even before the adoption of FPL53.
The design of the FPL53 triplet used more reliable and easily obtained glasses, and provided better color correction that it would have had using ZKN7.
Clear skies, Alan
I don't know squat about it but in the link it says: "The first EDT design used ZKN-7 crown glass and FPL-52 ED glass. The last runs of the smaller EDT models used the same crown glass but FPL-53 ED glass. They were even better corrected for monochromatic and polychromatic aberrations."
So, if two conditions are met:
1. The Back essay is accurate and
2. The phrase "same crown glass" in the second sentence means ZKN-7, which syntactically appears to be the case, than FPL-53 for at least "the last runs of the smaller EDT models" mated FPL-53 with ZKN-7.
I am simply reading the text here. I wouldn't know ZKN from my buddy Zeke.
p.s. actually I don't know anyone named Zeke.