Kirby being Kirby ends up laying out an additional 13 page story to follow, and Lee goes with Kirby again for #41. Thinking about this more, what if Iron Man like Hulk or Spider-Man at this time was going to get his own dedicated series? This would also make sense that #41, though a separate full length story, does not have a job number on the title page. Furthermore, instead of the standard 24 pages, this would then be 26 pages, possibly adding a 2-3 page blurb again in the beginning of issue #41 about Stark's many hats he holds. It could also be the addition of the opening splash page that inflated the page count.
Similarly, like The Amazing Fantasy series had the follow-up Spider-Man stories were slated for future issues in the same title, the series was canceled, and Spider-Man at the same cover date of 3/1963 is given his own title, where we see these stories again. Keep in mind, March 1963 by far was the highest number of titles with 9 instead of the usual 7. Was Iron Man then pushed to the Tales of Suspense series rather than his own full length title that same month 3/1963? Again, this theory is something we'll never be able to 100% confirm above, but it does make a lot of sense and why the gap between Heck's first and second Iron Man stories.
Heck on the other hand draws #39 and his second story, which thinks it will be either for issue #40 or #41, but gets pushed to issue #42! Yes, a little confusing, but I think you got it. Thus, Heck does Iron Man’s first story art with issue #39, presumably does his second story art for the next issue, only to find out Kirby gets the next 2 issues #40 and #41, and Heck’s story goes to #42, our subject art, which we'll discuss shortly.