The loathsome cover aside, this book seems to contain many of the names of illustrators who worked for the defender and many others besides. I wouldn't be at all surprised if our mystery artists were among them.
https://books.google.com/books?id=CeDeCwAAQBAJ&pg=PT71&dq=Black+illustrators+for+the+Defender&hl=en&sa=X&ved=0ahUKEwjD_onZ2KrOAhVHVh4KHW4cDCQQ6AEIIzAB#v=onepage&q=Black%20illustrators%20for%20the%20Defender&f=false
https://books.google.com/books?id=CeDeCwAAQBAJ&pg=PT71&dq=Black+illustrators+for+the+Defender&hl=en&sa=X&ved=0ahUKEwjD_onZ2KrOAhVHVh4KHW4cDCQQ6AEIIzAB#v=onepage&q=Black%20illustrators%20for%20the%20Defender&f=false