The unusual concrete structure you've encountered is part of the remnants of the USS Inaugural (AM-242), a former U.S. Navy minesweeper. After serving in World War II, it was eventually decommissioned and sold for scrap. The concrete portion you see once formed the ship's bow. It was salvaged and donated to the city of Alameda, where it was installed in Encinal Beach Park alongside other memorial elements dedicated to naval history. The number "66" is likely an arbitrary marker or a local designation, as the ship's hull number was 242. This unique repurposing of a ship's part offers a tangible connection to the Bay Area's rich maritime past.