For me, the bigger mystery was always Giant-Size X-Men #1. For years, the reproductions of the cover that I saw in various reprints all looked like this (click for a close-up, hi-res look):
The real cover looks like this:
There’s only one difference between the two: the cover date. For whatever reason, all the images that I had seen over the years had a cover date of May. That’s how the cover appeared reprinted on the inside back covers of X-Men Special Edition #1 and Classic X-Men #1 (which sports an awful re-coloring of the classic cover). It’s also how it appeared in Marvel Masterworks (the volume featuring Giant-Size X-Men #1 was first published in 1989) and the 1991 Marvel Milestone reprint that even included all the original advertisements of the original comic, and in the reprint in the first hardcover collection of Ultimate X-Men, which came out in 2002 or so.
But Marvel obviously also had access to the correct image, which appeared in 1988’s The Official Marvel Index to the X-Men #4, and in the 1994 update of that series. It also showed up correctly in the 1996 first printing of Essential X-Men Vol. 1.
So, where did this version with the May cover date come from, and how did it become the primary — but not only — version Marvel used? The original artwork — which can be seen here — includes none of the trade dress and offers no answer. My only credible thought is that a version was prepared for a house ad that might have appeared just before the issue came out. But I’ve not been able to find such an ad anywhere online, so it’s all just supposition on my part.
The May date is probably correct. X-Men #93, the last reprint issue of the series, had a cover date of April 1975 and X-Men #94 had an August 1975 date. The gap between Giant-Size X-Men #1 and X-Men #94 make sense, given the now well-known story about how the story intended for Giant-Size X-Men #2 was broken up into two issues and run as #94 and #95 when editor and writer Len Wein left Marvel. The May cover date also places the release of this issue in January or February of 1975 (I always go by my memories of the May Marvels coming out in the direct market in January, usually a few weeks ahead of issues showing up on newsstands). But looking at the actual indicia for Giant-Size X-Men #1 shows the only cover date to be 1975, and the frequency of the book as quarterly. Giant-Size X-Men #2 similarly only has a 1975 cover date, but the frequency had been bumped up to annual.
Anyways, the mystery of where the May cover date came from and how it became so commonly used by Marvel over the years is likely to remain a mystery.
The changes on X-Men #1 are in a lot of ways not as obvious, but definitely more significant. Here’s the real thing:
I recall reading somewhere – I can’t find the piece or remember where I read it — that the version with the grass background and power effect for Marvel Girl was part of the original artwork that Jack Kirby and whoever inked this cover turned in. Taking a closer look, it’s clear that more was changed between that version and the one printed than those elements just being dropped out.
A close look reveals that Marvel Girl, Angel and Beast were moved up and spaced out a bit, perhaps to make each more distinct on the cover. There’s also a few motion lines dropped over near Angel. It’s kind of horrifying now to think that this classic cover might have been cut up with an X-acto knife and the characters all re-pasted into their new positions in Marvel’s production department.
But it’s not that the original was changed that’s so much of a minor mystery as, again, how the non-published version was reprinted so often. Someone at Marvel, however, has noticed the difference, as it has been corrected in the revised editions of the Marvel Masterworks series to match the published version of the original comic.
Maybe someday, convincing answers will come forth and allow me to settle this errant thought. But if not, it's also fun to roll this completely inconsequential bit of trivia around in my brain every now and then.