An art historian recently spotted the 16th-century artwork in a rare photograph of Hitler’s old apartment that was printed in a 1978 furniture catalog

Cupid Complaining to Venus, Lucas Cranach the Elder, 1526-1527 National Gallery Picture Library

An overlooked photograph from an old German auction book has shed new light on the mysterious history of a painting at London’s National Gallery.

Discovered in a furniture catalog from 1978, the grainy black-and-white image—captured sometime in the early 1940s—appears innocuous at first. It features an upscale sitting room with a chandelier and matching furniture set before a fireplace mantle. On the far wall, angled slightly askew beside a tall door, a painting hangs.

But it is not just any painting. The artwork is Cupid Complaining to Venus , an early-16th-century masterpiece by German artist Lucas Cranach the Elder. Nor is this just any Munich apartment.…