Image circle of the 610/9 Apo-Nikkor is specified as 1030mm at f/16 - at 1:1 magnification, which is what it was optimized for. The 420/9 has a specified image circle of 710mm at f/16 - again, for 1:1 magnification. Image circles for infinity focus would be half that, so unless the lens gains a whole lot of additional coverage as you stop it down further, 20x24 is extremely optimistic for the 610 if you intend general purpose rather than exclusively close-up photography.

You might have a look at Dan Fromm's overview:

http://www.galerie-photo.com/apo-pro...ikkors-en.html

Ken Lee, at the Large Format Photography Forum, uses Apo-Nikkors and has spoken highly of their quality for distant as well as close-up work.