I found this link the most revealing....LMUS!
Monday, July 13, 1942
To save $2,000,000 worth of nylon, paper, ink and printing, Treasury employes will dust off some long-stored bundles of pre-Roosevelt Federal Reserve gold notes, put $4,200,000,000 worth into circulation.
They won't be, as promised on their face, "redeemable in gold on demand." Like all New Deal Federal Reserve notes, they may be exchanged only for "lawful money of the United States," i.e., smaller bills or coin. http://www.time.com/time/magazine/ar...#ixzz2LHrzwGxQ