Take a look at one of the banknotes in your pocket. The serial number
appears twice, at lower left and upper right. The prefix letter before the
number indicates which of the 12 Federal Reserve Districts the note is
printed for. The suffix letter indicates the block of numbers. When the
block is "full," the numbering starts again at 1 with a new block letter.
Thus hundreds of millions of notes can be printed for a given series date
for the 12 districts and whatever number of blocks.
The series date indicates only when the series started as the same series
date is used until there is a major change, such as a new Treasurer, so
notes may be printed with the same series date for a number of years. As
one example, the 1935 series, with different suffix letters, was printed
into 1957. That's why the 1935-G series notes come with and without the In
God We Trust motto.
Alan Herbert - Contributing Editor - Bank Note Reporter
Krause Publications - "The World's Largest Hobby Publisher"