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.