The eBay seller calls these "cartoon prints," but they look like model sheets to me so that's what I'm going with. Model sheets are references used by animators to keep the onscreen appearance of their characters consistent. They were generally printed in small, unspecified quantities for in-house use. I don't know how common it was to print on both sides of the sheet, but for Hanna-Barbera Productions to use that or any cost-saving measure doesn't really surprise me given the quality of their animation.
Hanna-Barbera Productions, "Dastardly & Muttley" model sheet, 1970
Hanna-Barbera Productions, "Dastardly & Muttley" model sheet, 1970
These early renderings of characters from Walt Disney'sSnow White and the Seven Dwarfs (1937) are most likely the work of animator Paul Sprunk. There are four original drawings and one printed model sheet of the seven dwarfs.