I believe I took the below text from Ebay seller "shamanbooks".
In the Bob Abbett cover editions all of the cover art was the same for all of the books, with the exception of the covers for Thuvia Maid of Mars. In this case there were two different covers. The first features Thuvia standing in front of a giant Banth. The second very rare cover published only once - in 1969 - and shows Thuvia being helped out of a flyer by Cathoris, Son of John Carter.
Apparently Ballantine Books commissioned Bob Abbett to produce 12 original covers for the ERB Mars paperback series after two additional John Carter adventures were discovered in ERB's archives. As it turned out there was only enough material for one book and this was published as the 11th and final book entitled 'John Carter of Mars. (Controvery surrounded this additional book since it was rumoured that ERB's son, John Burroughs, edited and wrote the ending for the story... something he denied for years, until very recently).
The Abbett series was originally published in 1963, with a second edition in 1965. In 1969 Ballantine decided to replace the cover of 'Thuvia Maid Of Mars' with the extra cover - now known as the rare 'Variant - to give it what was considered then to be a 'sexier' image.