This card set was not sold in packs, but was originally distributed through ads in the Famous Monsters of Filmland magazine, from Warren Publishing. Famous Monsters sold smaller merchandise directly to readers through its Captain Company ad sections, and apparently this card set was commissioned and sold in bulk to Warren alone, then distributed to collectors by mail order. (So that full sets can be found more easily than individual cards.)
Evidence for this is bolstered by the fact that, even aside from the name, the card backs carried advertisements for other Warren publications, such as Screen Thrills Illustrated magazine, or merchandise such as rubber monster masks and gloves. In addition, the distinctive Famous Monsters logo sat at the top center of every card back.
The card fronts showed black and white photos of period monster movies (such as the kind published en masse by Famous Monsters editor Forrest Ackerman) surrounded by an orange border, with the card title at bottom and the card number in a circle to the side.
Note that this set is similar to Rosan's Terror Monsters cards, as well as to the Monster Cards (also presumed to have been printed or distributed by Rosan) and Nu-Cards' Horror Monster Series. It is unknown just how closely linked these different card sets are in terms of printing, distribution, etc.
Note also that card fronts can be found with different advertising on the back, so that each card front may have (say) 4 different backs. This may cause some collectors to try to get each image with every back matter.