To follow up on what 20th Century Danny is continuing in his blog, I pulled out the original pages that I discussed in my comment on the subject at his blog. I wanted to post scans to illustrate more clearly what I was talking about, and what it is that makes the Avengers #1 splash so suspect.
You can clearly see here the red pencil used to note the issue number on the top margin here at the top of FF #20 page 14. I have no idea what "No. SPS" means.
Here, at the bottom of the same page, is the thick black marker used to denote the page number.
And here is the code seal on the back.
And now, Avengers #14, page 24. Interestingly, there is no large black number on the bottom of the page as the artwork has been trimmed, but we get the same sort of markings on the top part of the page, from book to page number.
Interestingly, there is no code seal on the back of the Avengers page, which I find rather interesting, considering its vintage. Perhaps trimmed off of the bottom?
and, ah! I see that Dan posted a new post with other scans from Avengers #1 on his site while I was typing this. Ah well, more data to mull over!