IIRC, this was a time-travel cross-over with Marvel Two-in-One Annual #1 which also featured the other WWII team, the Liberty Legion...and another kool Kirby cover, this time with just Ben Grimm and the Liberty Legion!Note: the FF tale was penciled by John Buscema, the MTiO story was penciled by his little brother, Sal.I think it's the only time the duo did a two-part story like that.
In Fantastic Four #102, Kirby was on his way out the door, but still did some amazing panels of Sub-Mariner.And remember that Iron Man/Namor battle he drew in like fifteen minutes to bail out Gene Colan? Amazing-er.
Cool beans, now I gotta check TIO Annual #1 out---intrest'n about the brothers Buscema---would you consider the alternating art on The Avengers in the same category? Only kept some of the Avengers, and only Big John issues, but your observations made it echo
That short sequence of Iron Man/ Subby is some of my favorite Kirby, with a heavy side of testosterone, it's kinda reminiscent of Thor/Hercules