Where is the series letter on US paper money?

Answer 1

If there is one, it's next to the date.

However, it's confusing because the first printing of any series doesn't have a letter. The second printing has an A, the third has a B, and so on.

And if that's not enough the reasons that a series letter is added have changed over the years. Up till 1974 a new series year normally started only when there was a major redesign. The series letter would updated when a new Secretary of the Treasury or US Treasurer was appointed. That caused some bizarre situations because bills weren't redesigned very often in the mid-20th century. The 1935 series of $1 bills was printed all the way to the 1960s, and series letters got as far as "H".

In 1974 a new policy was adopted. A new year series would be started whenever a new Secretary took office, and the letter would be updated when a new Treasurer was appointed. That means the time between new series years is usually short, and series letters rarely go above B or C. On the other hand, the new policy has some oddball results as well. For example when a new Secretary was appointed in 1988 no $10 bills were printed until there had been a second change in the Treasurer's office, so there are 1988-A $10 bills but no 1988 bills. Similarly, when the new purple and gray $5 bills were issued in 2008 the same people occupied both offices as in 2006, so both the old green and new purple bills were issued in the same 2006 series without a letter change.