It was vague and didn't surrender plotting credit to Kirby, but didn't clearly give him the credit he wanted, so it may have been part of his eventual separation from Marvel.

Lee, Kirby and Sinnott were all important to the process, and it's unfortunate that fans feel a need to pick at it until it has gotten to the point of "Stan vs. Jack" for many of them.  The whole process worked best with Lee, Kirby and Sinnott - no matter who did what to whatever extent.

That's not correct, unless you are specifically meaning the phrase "produced by Stan Lee and Jack Kirby." The credit boxes Stan used started at least a couple of years earlier, around 1964, and the earliest Fantastic Four comics had "By Stan Lee and Jack Kirby" written on a page somewhere.

Huh... Look at that....

1962

I can find more examples, if you like.

I should add that to those of us actually familiar with the original comics, and not simply relying on a quick Google search to inform us, the credit "By Stan Lee + Jack Kirby appeared on most of their collaborations, even before "Fantastic Four" #1, in the monster comics, before Stan formalized a credit box that credited not only his artists, but the letterer and colorist.

Stan's detractors want to continue to put forth that he never gave credit, which is beyond ridiculous in the face of the evidence. You can argue he didn't specify EXACTLY who contributed what to a creation in every instance, but he definitely credited his partners.

Total nonsense.

Back in 1961, neither Stan nor Jack had any clue how big their creations at Marvel would become, yet Jack was still credited alongside Stan in most comics they did. It wasn't until Marvel started to become a big thing towards the later end of the decade that it appears Jack was becoming unhappy with the media attention that made it appear Stan Lee was the sole creator of Marvel's characters. And that was more the media, which typically simplifies and glosses over facts, than anything Stan Lee did directly. Frankly, I think Stan Lee has gotten all the flack because the media would not mention his partners, and blamed him for that.

As I'm sure you're aware, back in the early days you would occasionally (and more frequently in the '60s) see the front splash signed with the collaborators names (like on that Incredible Hulk page).  But I wouldn't call those credits as we came to know them.

There may not have been a formal credit box on page one as with the page below, but there was certainly credit given to both Stan and Jack on Matt's page from the Hulk. Are we discussing if credit was given at all or if credit was given in a formalized box?

If I were a new reader and picked up Incredible Hulk #1 I wouldn't have had any idea what either of them did.  Stan's Soapbox, letters pages and credit boxes (among other things) really gave us a lot more information on the process than was available to most fans before.