I have spoken to the seller, who is enormously reputable, and a hugely respected and well liked player in the vintage game, and he tells me he has seen this exact dial several times in his career, and that he is 99.9% certain the dial is original. (I'm not God, he laughed, but...) From the tone of his voice and reputation, I tend to believe him. He agreed that the seconds hand is not correct and agreed to have it changed, he also agreed to install a correct star crown.
At this point, it would be hard to argue against the watch's merits and authenticity, I think... but... still I'd like to hear your thoughts.
There seems to be zero doubt that Zenith did indeed manufacture 135's with black dials--they are much rarer than the silver, but absolutely were manufactured. The fact that the word zenith is elongated is quite possibly a style difference, as watchmakers are prone to do... as you can see from the FL version in above-post's photo, it appears that black dials were sometimes this way...
(I think no one can be 100% certain, do you? We try our best to answer these questions, but with a rare example, we just do our best from knowledge and past pictures.)
Does anyone have a Zenith history book?
I'm feeling very good about this watch. What do you think?