Does anyone have any idea why these two cards have red and black variants?

One theory I've heard is that the red ink started to run out late in the print run, causing later cards to show up with black names -- does that sound plausible?

I'm also told that no other players in this set are printed with their names in two different colors -- is that true?

I don't know the real explanation, but running out or red ink seems unlikely. Every card on the printing sheet uses a combination of red (magenta), blue (cyan), yellow and black ink. As far as I know the Hill and Rentzel cards are the only ones with both variations of red and black names.

Football cards in general, however, have never been thought of too highly by the general collecting public, as well as catalog publishers, so it wouldn't surprise me at all if new variations are discovered in this set, or any of the older football sets. They just never seemed to get much respect.