Sproul, Allan, 1896-1978. Letter to Mr. William McC. Martin, Box 16, Folder 2, William McChesney Martin, Jr., Papers. https://fraser.stlouisfed.org/scribd/?item_id=456847&filepath=/files/docs/historical/martin/16_02_19500714.pdf&start_page=1, accessed on October 17, 2017.
Regarding ECA and British balances.
- Board of Governors of the Federal Reserve System, Federal Reserve Board
- Meltzer's History of the Federal Reserve - Primary Sources