The Ruins at Cerros

The Lost Mayan Ruins was an exclusive memory in Assassin's Creed III, only available to those who pre-ordered the Freedom Edition of the game, or at Gamestop.

Completion of the memory awarded the Assassin Connor Kenway with the Sawtooth Sword.

Source

 * Game Informer: Assassin's Creed III Gets Three Collector's Editions In UK