I keep getting confused. Could use some help. For U.S. banknote identification, such as:
Series 1957 A - (B/A)---(P-419a)
The letters in parentheses. Which letter corresponds to which marking on the note (i.e. in the example above, B = ? and A = ?).
Link to example of a note in question:
https://en.numista.com/catalogue/note201794.html
THANK YOU!