Hard to tell age on something like that. In your "tap and die" post you said that trucks drive on the beach. Probably just a lost shackle from one truck pulling another out of the sand. I used to carry several shackles, chains, snatchblock, and straps in my Jeep just for that purpose. Pretty common site at Pismo in California to see trucks pulling each other out of the sand.
Very common. Modern galvanized. I have a couple in my truck. Good for attaching trailer safety chains when towing. Probably got lost during the beach renourishment.