SPOT THE DIFFERENCE...

Copyright MARVEL COMICS

Above is the cover from the second THOR KING-SIZE SPECIAL from 1966, below is a reprint from 1994 - which still carries the original indicia, but with the words 'second printing' appended to it.  Sharp-eyed readers will notice that ol' Goldilocks' right arm is different on the two versions, the first having been amended before printing, the second featuring the cover as KING KIRBY originally drew it.

The reason for this is pretty obvious: Marvel's file copies of artwork were usually photocopied before any alterations were made, hence many reprints down through the years sported pages as they'd originally been drawn, not published.  Thankfully, in recent years, Marvel has addressed this issue and striven to ensure that reprints are as close to archival quality as possible.

Thought you might enjoy seeing Kirby's original version.

But hold!  There's even more to the story!  Take a good look at this third version of the cover, below.

While thumbing through my hardbacked volume of THOR MASTERWORKS Vol 5 recently, I espied this amazing oversight.  Either the patch covering Thor's original arm has fallen off (which would mean that Marvel had access to the original art), or an art restorer re-created the new arm on a file photostat, but then simply forgot to delete the original Kirby limb it was intended to replace.  Who knows the truth behind this Marvel mystery?  Answers in the comments section if it happens to be you.