The Fed was one of my accounts when I worked for IBM. The Federal Reserve System is made up of 12 twelve districts. One of the responsibilities of the Fed is to distribute currency to the banks in their district. Federal Reserve notes have a letter, from A to L, corresponding to the district it was issued for.

01-Boston (A)
02-New York (B)
03-Philadelphia (C)
04-Cleveland (D)
05-Richmond (E)
06-Atlanta (F)
07-Chicago (G)
08-St. Louis (H)
09-Minneapolis (I)
10-Kansas City (J)
11-Dallas (K)
12-San Francisco (L)