Bill, you are correct as far as the original question. The Gurney/Foyt MkII entry said "Mercury" in script within the rocker panel stripes, as opposed to the normal "Ford" (and I may have misstated the facts in my original question, as it sounds from your answer as if at least one of the other entries was also badged as a Mercury). Perhaps the earliest known example of "badge engineering" in the industry?
As far as the other half of the question, I'll leave that one to Big Bob. All I can say is, it was not a good year for that GT40...
Bob