Medieval Guy wrote:
DNAlien wrote:
In most cases reversion starts 56 years after the work was created, and lasts until 70 years after the death of the author.

I don't think that's true. I think there's a pretty narrow window (a few years) when you can file for reversion.

I think you're confusing the term of the copyright with the window during which they can reclaim it.
There's a period of time when they can file (which is the period we're in right now for Kirby's Fantastic Four and others), but the period the claim is relevant to is the one after 56 years, after the original terms at the time of creation expired (there was also a renewal at 28 years, but anything covered by that has long passed).  Even if the Kirby heirs had been successful in their claim (or are successful in appeal, fingers crossed), Marvel would still have the FF until 2017.