Newbie Question Of The Day.

I mostly collect coins but have a few US dollars and a pretty large world currency collection. I recently purchased a PaperMoney Values price guide as it was talking about military notes. Looking at the price guide I was trying to look up some of my bills. I know the series is the date issue, ie 1964, 2003 or 2003A. The number is talking about the Friedberg number that you can get from the Guide Book of United States Paper Money. What is the alpha character after the Friedberg number? Does the alpha correspond to the alpha letter before the serial number on bills?

Thanks for the help.