The 1942-S Shilling was struck at the San Francisco Mint with a total mintage of 4,000,000. In the same year an additional 1,380,000 pieces were struck at the Melbourne Mint. The San Francisco type can be differentiated from the Melbourne by a small 'S' mint-mark located below the ram on the reverse design (see image below). The large mintage means that the type is usually affordable even into mint-state.
The high mintage of shillings in 1942 was a response against a major shortage of both silver and copper coinage. One of the key reasons for the shortage was the mass arrival of United States soldiers into Australia from the Middle Eastern theatre. These troops, along with a general increase in public spending, created an unprecedented demand for coinage. The Australian mints' simply could not make supply match demand so a deal was struck with the United States under President Roosevelt's Lend-Lease Policy. It was agreed that the United States would provide Australian with a substantial quantity of silver coins and in return Australia would return an equal amount of silver after the war.
President Theodore Roosevelt had created the extensive Lend-Lease program in 1941 as a way to assist allied forces during the Second World War. The program allowed the United States to provide war materials including but not limited to - ammunition, aeroplanes, and food - that could then be repaid for, "...in kind or property, any any other direct or indirect benefit which the President deems satisfactory." The program proved essential to the allied war effort with the total value of the aid reaching $49,100,000,000. (Encyclopaedia Britannica, 2017)