I'll be blogging with fewer resources for awhile—I've moved and my Mac, with Photoshop, will take time to follow me. My PC laptop doesn't even want to unzip files. So, with a couple of images I already had on the PC:
Ever since fandom has been indexing, the Classics Illustrated and World Around Us covers by this artist have been credited to Doug Roea. It certainly looks as if he signed his name that way; the E would be a stylized one without a downstroke. (Click to ensmallen; thanks, Blogger.)
However, there was a paperback cover artist at the same time who didn't sign his covers, that I've seen; he did a few early Doc Savages at Bantam: The Land of Terror and The Lost Oasis. But he was credited on the copyright pages at Avon on the Tros series and the A. Merritt fantasy novels, and on the back cover of Lancer's Magnum imprint publication of H.G. Wells' The Island of Dr. Moreau. His name was given as Doug Rosa.
Occam's Razor—the simplest solution is the best—would suggest that instead of two Sixties cover artists, Doug Roea and Doug Rosa, there's only one: Rosa, whose name the paperback publishers didn't have to decipher from the signature. The supposed stylized E without a downstroke on the left could be a stylized S without downstrokes on the upper left and lower right.
UPDATE: Jake Oster sent me a link to a painting by Doug Rosa, done for the Marine Corps, with this signature. Case closed!
That would make two D. Rosas in comic books.