The 1943-S Threepence was struck at the San Francisco
The very large mintage of threepence in 1943 was a response against a major shortage of both silver and copper coinage. One of the key reasons for this shortage was the mass arrival of United States soldiers into Australia from the Middle East theatre. These troops, along with a general increase in public spending, created an unprecedented demand for coinage. The Melbourne and Perth Mint simply could not make supply match demand so a deal was struck with the United States. Under President Roosevelt's Lend-Lease Policy the United States agreed to provide Australia with a substantial quantity of silver coins. Australia guaranteed to then return an equal amount of silver after the war.
President Theodore Roosevelt had created the extensive Lend-Lease Policy 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, or 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)