I checked my collection and fortunately I didn’t have too many of those. And while I didn’t know the term Cinderella stamps then, the few non-postal stamps that I had were segregated out in my collection.
While interacting with other philatelists on a Facebook group, I discovered Cinderella stamps. In philately, a Cinderella stamp is “virtually anything resembling a postage stamp, but not issued for postal purposes by a government postal administration” The term also excludes imprinted stamps on postal stationery. You can read more about these here: https://en.wikipedia.org/wiki/Cinderella_stamp