Wow, this really isn't making sense, unless the Moneyfactory.com got it wrong. Here's what I found on the Treasury's Burau of Engraving and Printing web site about currency series.
Series Year and Federal Reserve Bank IndicatorsA new series will result from a change in the Secretary of the Treasury, the Treasurer of the United States, and/or a change to the note's appearance such as a new currency design. After the Secretary of the Treasury changes, a new series year is adopted (e.g. 1999). When the Treasurer of the United States changes, a suffix letter is added to the current series year (e.g. 1999A). Additional changes of the Treasurer, whereby the Secretary of the Treasury remains the same results in subsequent letter changes to the current series year (e.g. 1999B, 1999C, etc.).
source: Bureau of Engraving and Printing
OK, here's what we know 1. Humphrey was the Sec of Treasury from Jan 20, 1953 - July 29, 1957Therefore, currency printed during his tenure should bear the series year 1953.
2. Priest was Treasurer of U.S. from Jan 28, 1953 - Jan 20, 1961Priest's term began 8 days AFTER Humphrey's tenure as SecTreas. If any bills were printed in the 8 days between when Humphrey started and Priest arrived, they should bear the series 1953 and have Clark (Priest's predecessor) and Humphrey's signature. I have posted an image on page 3 showing my bill is a 1953, yet the signators are Priest/Humphrey.
Once Priest began serving as TreasUS under Humphrey, new printed paper currency should have been designated as the 1953-A series and the signators should have been Priest/Humphrey. Again, my $5 bill is a 1953, but the signators should suggest it is a 1953-A.
3. Anderson became Sec of Treasury July 29, 1957Since the SecTreas changed, a new series year should be adopted when Anderson arrived and they should bear the series year 1957. Since Priest was still TreasUS and he and Anderson resigned Jan 20, 1961 currency with their signatures should be 1957 series (no letter suffix).
As an aside (for those who haven't seen a pattern), entrance and resignation dates for new officials at this level (not surprisingly) coincide with, or are very close to, the inauguration day of a new President.
So if the BEP web site is accurate, why is my bill a 1953 series and not a 1953-A, and why is Angelo's bill a 1953-A and not a 1957?
My guess is that either I misinterpreted the explanation of how series years/letters are assigned or it's inaccurate (perhaps by ommission).