The Marabout covers reused the art of Bantam paperbacks (mostly James Bama). Marabout incorrectly attributed several early cover pictures to James Bama. 

On several editions, Marabout inverted the right and the left of the Bantam picture. They made Doc look to the right when it was not the case. The reason for doing so is unknown (perhaps aesthetical reasons). 

Types 7 to 9 display a scaled down version of the picture, within a white frame. The logo is of straight type. 

Type 7 to 9 cover picture 

Type 7 can be distinguished from other types by the missing number 
within a yellow/brown circle on the book edge. 

Type 7 book edge

Type 8 and following book edge

Each series within the Marabout Pocket collection had its own color. Bob Morane was red, Jo Gaillard was blue, Doc Savage was brown. The actual color of the circle on the Doc Savage books edges is quite varying. It ranges from light green to dark brown, passing through several yellowish tones. The exposure of the books to bright light can account for these variations. 

Type 8 and 9 can be distinguished from each other by the color of pages. They are white for types up to 8 and cream for later types. This difference can not be shown on scanned picture and can be hard to tell if you do not have a book from the other type to compare. 
Marabout books with white pages (type 7 and 8) are very fragile. Apparently the glue that was used has degraded through the years. Although they may look in good shape, the first attempt to open them causes the separation of block of pages from the cover. Putting some new glue can be used to repair them. 

Type 9- are 1969 reprints of earlier novels. The original covers (type 7 or 8) are reused, but the inside of the book is a new reprint (cream pages). You can recognize a type 9- by the color of its pages and by looking at the back issues list, which displays other Doc novels that are more recent than the book itself (time paradox!) 

Types 11a and 11b display a full size version of the picture. the logo is of wavy type (like Bantam). 

Type 11a cover picture

Type 11a and 11b can be distinguished from each other by the position of the Marabout trademark: directly on the cover picture for types up to 11a (see above) and within a colored circle for type 11b. 
Types up to 11a have "dull" covers whereas type 11b have shiny covers. This difference can not be shown on scanned picture. 

Type 11b cover picture

This picture comes from the special movie edition reprint.