The "Linen Card Era" of Post Cards was from 1930 to 1945.  Most World War II Post Cards were printed on this linen type paper stock. Soldiers would send these mementos home as proof of "being there" in the case of attending one of the camps or forts depicted in the cards.  They were an inexpensive (if not free) way to correspond with loved ones or friends back home or around the world.

Prior to the publication of the June 1945, Manual of Military Occupational Specialties NAVMC 1008-PD, the Marine Corps generally followed the Army's system for classifying jobs for personnel. As stated in the manual, the Marine Corps often used Army publications [1] for determining military SSN's [2]. After the publication of this manual, it would be the sole reference for classifying military personnel in the Marine Corps. The Army Manual, TM-12 426 would still be used for civilian personnel.