If the Fed is really a "private" bank in a capitalist system then it wouldn't have a "monopoly" and instead would be one of other competing factors for currency.
Centralization of credit in the hands of the state, by means of a national bank with state capital and an exclusive monopoly.
The Federal Reserve System, created by the Federal Reserve Act of Congress in 1913, is indeed such a "national bank" and it politically manipulates interest rates and holds a monopoly on legal counterfeiting in the United States. This is exactly what Marx had in mind and completely fulfills this plank, another major socialist objective. Yet, most Americans naively believe the U.S. of A. is far from a Marxist or socialist nation.