from
Bouvier's Law Dictionary, Revised 6th Ed (1856)
LETTER OF ADVICE. comm. law. A letter containing information of any
circumstances unknown to the person to whom it is written; generally
informing him of some act done by the writer of the letter.
2. It is usual and perfectly proper for the drawer of a bill of
exchange to write a letter of advice to the drawee, as well to prevent fraud
or alteration of the bill, as to let the drawee know what provision has been
made for the payment of the bill. Chit. on Bills 185. (ed. of 1836.)