A recently discovered book annotation allows a unique and spectacular coin of the Roman emperor Severus Alexander, minted by the city of Cyzicus in Asia Minor, to recover its provenance. It belonged to the collection of Sir Hans Sloane, which was to become the founding collection of the British Museum, where the coin can still be found. The MS catalogue of the Sloane collection was lost in the Second World War, so we depend on chance finds such as this for its reconstruction.