Source: Board of Governors of the Federal Reserve System (US)  

Release: H.4.1 Factors Affecting Reserve Balances  

Units:  Billions of Dollars, Not Seasonally Adjusted

Frequency:  Weekly, Ending Wednesday

This data series has been discontinued, please see series ID: https://fred.stlouisfed.org/series/WMTSEC1
Memorandum item: Securities Held in Custody for Foreign Official and International Accounts - Marketable U.S. Treasury Securities. For further information
on the changes to the H.4.1 Statistical Release please visit the following: http://www.federalreserve.gov/releases/h41/