The Fed's website has a FAQ "Is U.S. currency still backed by gold?". Part of the answer writes:
The Congress has specified that Federal Reserve Banks must hold collateral equal in value to the Federal Reserve notes that the Federal Reserve Bank puts in to circulation. This collateral is chiefly held in the form of U.S. Treasury, federal agency, and government-sponsored enterprise securities.
In 2017, there were \$41.6B in notes in circulation (source). (Of course, the Fed's total assets vastly dwarf this figure -- they are around \$4T now, but were over \$700B even in the years before the financial crisis.)
Three questions about the above statement:
The statement presumably refers to some specific law passed by Congress. What was this law and when was it passed?
Does the Fed specifically earmark some portion of its assets as "collateral" for the notes in circulation? For example, in 2017, did the Fed have \$41.6B in assets specially earmarked as "collateral" for the \$41.6B in circulating notes?
Does the Fed actually keep any record of this? (This was one thing I was looking for but unable to find.)