Grosset & Dunlap purple/white spine formats
I'll admit, in this section, I'm lumping a few formats into one. James Keeline (correctly) separates these picture cover formats out into different formats / variations. There were variations, as James shows on his page (listed under Further Resources).
Although all titles still in print in the '70s were eventually available with this purple/white spine, the earliest printings of #71 and #72 were not printed in this format (until the mid-'80s)--originally they were published with solid purple spines, as seen on the Solid-Purple-Spine page.
Variations of the rear covers in this picture-cover format, with the earliest to the left:
James Keeline provided this list he created regarding these printings (which ones were printed in this format, and other characteristics about them):
There is also an oddity that I just came across--I don't know how widespread it is: This is a copy of Sun Moon Cruise (#68) without the number on the spine OR the front cover.
Below IN the full list is a picture of another partially similar oddity: #67 without a number on the front cover as well.
Any blank spots below are simply because I don't have the book in my collection. If it has a spot below, it *does* exist.