US military payment certificates Standard catalog listings

Why are MPC not listed in the World(Pick) Catalog(s).?

The standard reference for those is, of course, Fred Schwan's Comprehensive Catalog of Military Payment Certifcates. Perhaps those have been considered specialty issues not to be included in the SCWPM.

U.S. MPCs are listed in Vol. 3 (Specialized Issues) of the SCWPM. But, Fred Schwan's "Comprehensive Catalog of Military Payment Certificates" is a more comprehensive and detailed listing.