According to the Secret Ending Guide from IGN , the relics must be placed in specific coordinates on the mural (e.g., C1, G2, K2) and turned in a precise direction (Left or Right) to open the vault. Key Quest Highlights: "Atonements"
The 50th "proper piece" is found during the final mission, Atonements , inside Emmerich Voss's tent at the excavation site.
To unlock the game's secret ending, you must collect all hidden throughout the game.