Actually, speaking as somewhat of an expert on this subject, cup size
nomenclature was still in its infancy at the time, and did not follow our current alphabetical system, which attempts to convey a physical image through a multi-step intellectually abstract conversion process.
Back in the day, cup size
was more of a direct volumetric reference, and included the half-pint, pint, quart, half-gallon, and (rarely) the gallon. The cup was thought to be hardly worth mentioning.
(Note the etymology of the word gal