On a lighter note, this letter from John Steinbeck to Marilyn Monroe made me laugh — though it's unconfirmed if it's real (it was found in Monroe's possessions, though!).
They should bring those back, be good for a little boost on a long day (couldn't be any worse for you than chugging a couple redbulls...)
They are the initials of the letter writer and the initials of the typist.