Complex sentence: Cinderella stamps are not normally listed in the main stamp-collecting catalogues; if they are, it is usually in a separate appendix within the publication, and so they are sometimes called "back of the book" stamps.
Split into simpler sentences: Cinderella stamps are not normally listed in the main stamp collecting catalogues. If they are, it is usually in a separate appendix within the publication, and so they are sometimes called "back of the book" stamps.