Thanks to John S. for his comments on this post: Captain America Spread on One Artboard.
That Cap double-pager is a real mystery. My best guess is that it’s an unused spread intended for pages 2 and 3 of Captain America #200. I think Kirby often drew his two-pagers on a single sheet (which may have been cut in half afterwards), and if he decided, before sending it in, to replace it with the two pages that were ultimately used, that would account for the lack of any stamps or hand-written demarcations on the artwork. I don’t see any traces of erased dialogue on it (which, if there, would confirm its origin as two pages intended for an actual story), but that could simply mean that Jack decided not to use it before he dialogued the sequence. And I agree with your guess that he must have liked it enough to have Mike Royer ink it later as a stand-alone piece — as he also did with an unused double-page spread from the Silver Surfer graphic novel (although that one was clearly drawn on — or cut into — two separate sheets, as seen in The Art of Jack Kirby, where the inking on the unused pages was wrongly attributed to Joe Sinnott).