It did get mentioned inside, but at that time it was still an ethereal dreadnought. The section on illusions/phantasms, on page 19, says: "An illusionist creates a phantasmal force of a large ethereal dreadnought (see the cover of the book)." When the artwork was reused for 1991 AD&D Trading Card #261, it became an astral dreadnought. As far as I know, it is one of only two creatures given statistics for the first time on one of the Trading Cards.Yep, the Astral Dreadnought's first appearance was, I believe, on the cover of the AD&D 1E Manual of the Planes (which ended up being rather odd, as the creature wasn't mentioned at all in the book's interior).