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DDI purchased about a dozen Fleer produced uncatalogued comic books in a Mastro auction awhile ago. CGC (grading company) said they would grade the Captain Marvel shown below, for sure, but not the others. All were produced by Fleer, but never distributed.
Uncatalogued comic books, from the 20th century anyways, are tough to come by; Overstreet has been a cataloguing force since the early 70's, and has constantly expanded it's coverage, and definition of what gets catalogued.
Topps Vault has unleashed a slew of test and prototype cards, including for major distributed sets. Fleer archives are known to survive in bits and pieces, and I specifically remember them opening a lot of what they had left to auction in the mid 90's.
Point being, uncatalogued examples are out there. I'll bet it's real.
Here's some comics, as an example:
CGC said they do not grade newspaper comics (understandably so), but I could not convince them the Playmate comic is not from a Sunday paper, but a separate issue.