Thank you, also here is a bit more info I found on the letter A under the date 1988

The letter "A" under the 1988 represents the second series of notes made in 1988 as there was a change of U.S. Treasurers after the first 1988 series had been printed. With the change in U.S. Treasurers/signatures during the same year, the second series printed that year had a letter "A" added below the year to indicate that change.