During WWII there was a shortage of metal and beer can production and sales slowed quite a bit. Also, any tin cans (1936-1942 beer cans) that were salvagable were collected during the WWII tin drives and reused. So fewer examples of the pre-war cans are available.
During WWII certain beer cans were produced exclusively for military use. These cans have the statement "Withdrawn Free of Internal Revenue Tax for Exportation" and were painted a dull (non metallic) green. Some olive drab labels were made in low profile & J spouts early during WWII but these types of cans were discontinued during the war.
Most of these cans were exported out of the US and are not readily available to collectors. Crowntainers from 1943 to 1945 were either olive drab or a non-metallic matte gray color. I have found some excellent examples of these cans in Europe (a basement in Austria!)