This cover was mailed from Snapper Creek, FL (Mar 30, 1976) to San Diego, CA at the domestic first class postage rate (25¢). The Snapper Creek post office is located in a suburb of south Miami, FL near the creek that originates in the Everglades and empties into Biscayne National Park Bay.
The Tortuga Local Post was a private delivery for a Miami, Florida resident taking mail to the local post office. The cancellation device includes an image of the local Snapping Turtle (do NOT put your finger in front of this guy’s beak). The image on the Local Post stamp is of an Archelon missing a flipper and is taken from a well known line illustration appearing in various classic fossil books. The value of the issue (500) is denominated in ‘terrapins’, a fictional monetary instrument. In actuality, no payment exchanged hands for the service of transporting the mail to the Snapper Creek post office.
The U.S. Postal Service dating device is a modern single ring. The private post dater is a pictorial four bar killer device commemorating the First Day of Issue for the Local Post stamp.