BEP Production Numbers Series/Style Designation

I do not know what these particular letters stand for (why these letters were chosen resp.) either.

The "N" indeed stands for the non-color designs of the 5, 10, 20, 50, 100 notes that were used between 1995 and 2013.

Diving into the deep end of the pool I see right away... most people are looking at other info besides the notes series you are asking about on the new style monthly production reports, here is some general information on the other info the monthly production report provides for those that are interested (such as april 2019 was referenced in the article) https://www.coinworld.com/news/pape...r-early-2019 which of course is referencing the monthly reports found here: https://www.bep.gov/currency/produc...tion-reports

As far as the notes series letters, the $100 was changed to the series of 2009, so called "next generation" notes, in May 2013 which changed from 'N' to 'G' along with all other next generation notes which use G in the notes series as described more in detail by SteveinTampa already. The $5 looks like it was changed in the series of 2006 which had both the old and new design in Oct 2007 for example. https://www.uscurrency.gov/sites/de...ochure_0.pdf SA is only used on the $1 and $2 notes and have less of the security features found on other denominations detailed in that brochure.