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, Foreign Official Assets in U.S.: Treasury Securities (DISCONTINUED) [BOPIOGTN], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/BOPIOGTN, February 5, 2016.