I wasn't suggesting Kirby had an exclusivity contract with Marvel, but simply that he wasn't able to get work elsewhere at the time (with apparently the exception of Classics Illustrated, which I hadn't known about). My understanding is that if a creator is working regularly for a company and creates something with the expectation that it will be published by that company, it is considered work for hire if the company does indeed publish it. Obviously, Kirby didn't think he could possibly sell the Fantastic Four to Classics Illustrated, so the fact that he was selling a bit of work to them at the time isn't relevant. Even if we accept the premise that Kirby came up with the FF entirely on his own, the only place he could reasonably have expected it to be published was Marvel. Kirby himself says he came up with the Fantastic Four specifically for Marvel to publish; he never mentions the idea of trying to sell it to someone else.