This series has been discontinued as a result of the comprehensive restructuring of the international economic accounts (http://www.bea.gov/international/modern.htm). For a crosswalk of the old and new series in FRED see: http://research.stlouisfed.org/CompRevisionReleaseID49.xlsx.
Source: US. Bureau of Economic Analysis
Release: U.S. International Transactions
US. Bureau of Economic Analysis, Other Foreign Assets in U.S., Net: U.S. Currency (Discontinued Series) [BOPIPUN], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/BOPIPUN/, September 3, 2015.