Quick question about U.S. notes

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!

Would you mind linking the note? I need some context.

Verweis : "drdrew"​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!

​The first letter is the prefix at the front of the serial number. The second letter is the suffix at the back of the serial number.

Thank you very much!