Howdy, I had a recent conversation with Jonathan Tweet who kindly pointed me out something I was not aware of.
Do you remember the knight on the cover of Vol.1 Men and Magic from the woodgrain set which was 'borrowed' from an issue of The Shield?
Well, Jonathan reported me that even the dragon of the Vol.2 Monsters and Treasures is not original: it is an image 'borrowed' from a childern's book... he told me that the image is cropped since it was originally cropped in the book as it was in a corner of the page. Unfortunately Jonathan cannot recall the title of that book (it was at his parent's house bitd) and has not further clues except for the fact he recalls it was a collection of illustrated fairy tales.
Do you know this story? Did you already find out the title of that book?

Although it's not an exact copy, there are a lot of similarities to the illustration "Es war ein ungeheurer drache.", or "It Was an Enormous Dragon", in the late 19th-century illustrated German folklore and legends compendium, Germania's Sagenborn.  Perhaps some of the characteristics were inspired by this illustration and others.