More than a few things have been strange about the large, late Roman-era sarcophagus first unearthed from the grounds beside Britain’s famed Westminster Abbey over 200 years ago. Three lines of Latin, deeply engraved into its limestone exterior, claim that this ancient sarcophagus once entombed Valerius Amandinus, an obscure figure apparently from Rome’s ancient outpost of Londinium—but sometime in the 11th century his body had clearly been replaced.

The first clue was the incongruous lid atop Amandinus’s 4th-century burial container: a single, rectangular slab of sedimentary oölitic limestone tapered in late Anglo-Saxon style and chiseled into a relief-carving of an early medieval cross.…