Better late than never answered:
Postal Notes are one of the more obscure postal issues serving as a type of money order form intended to facilitate the sending of small amounts of money through the mail. The intent was to eliminate the sending of coins or adhesive stamps through the mail. Postal notes could be issued only in amounts less than $5.00. An imprinted fee “stamp” on the face of the note was charged for each note.
Your postal stamp can be seen here in its original position.
Don in SoCal