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.
Tortuga Local Post First Day of Issue
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.
The Mesozoic Times 
Not really understand the meaning of “Local Post stamp”, but the cover looks very nice to me.
Hobbyist Local Posts have been popular in the US for some years and are an offshoot of older private posts. Anyone can have one as it’s a private carrier to move mail from anywhere to a government post office for mailing – as short as a walk to the nearest street mailbox or a drive into town from a rural location. Local post owners most often design and print their own stamps and have cancellation devices made as well. There are regulations requiring certain features to comply with government mail rules and these are published. A Local Post collectors society also exists for collectors interested in these issues. Hope this brief overview helps, check Wikipedia for more info.