Note Series A Question

unlike coins, (which are dated in the year they were minted), u.s. notes are dated by design series. whenever the design is changed, the series year is updated. sometimes, instead of changing the year, a letter is added. series 1935 silver certificates goes all the way up to 1935H...even though they are printed with "1935", 1935H notes were printed/introduced into circulation in the early 1950's.

for modern $1 bills, the only design difference between a series 2003 and a 2003A is the signatures. 2003 has treasurer marin/secretary snow, 2003A has treasurer cabral/secretary snow. series 2006 just came out, and they probably changed the date instead of using another letter because the design change was bigger. the series 2006 signatures are treasurer cabral/secretary paulson- two completely different names than series 2003's marin/snow. the next time one of the names change, you'll probably see a 2006A; the next time both names are different than 2006, you'll see a new year.