In The Unknown Diamonds, Dr. Shultz was a German research scientist who maintained a research station in the deserts of Southwest Africa (now Namibia), near the Grosse Kara mountains, where he collected monkeys to be shipped to zoos and research laboratories around the world. Because of the remote location, he was also licenced to operate an aircraft, an ex-wartime Dornier, flown by his pilot Leffers.
During his time in the desert, the Kalahari bushmen began bringing him diamonds which they found in the desert gravel. Shultz traded these diamonds for cheap trinkets such as pipes and jackets and also supplied them with tobacco and jerry cans of water. Shultz then had these diamonds surgically implanted into his monkeys which were then sent by air to his brother, also named Shultz in Britain where the diamonds, smuggled in free of duty, were sold and the proceeds donated to a charitable foundation in Germany caring for disabled soldiers.
Biggles visited Dr. Shultz's establishment while investigating the appearance of unknown diamonds which had been found at a pawnbroker's shop in London. Dr. Shultz showed him a batch of monkeys he was about to ship to Europe. Biggles, examining the animals, discovered lumps in the neck of one with a grey face. Thinking he had uncovered the details of the scheme, Biggles arranged for the grey-faced monkey, to be intercepted at the port of entry at London Airport where his brother Shultz was arrested and confessed to the whole scheme.
Dr. Shultz and Leffers were never seen again. They had fled by the time the Southwest African police reached his establishment. It was assumed that he and Leffers flew to Germany in the Dornier, either because they took fright after Biggles' visit or they had received a warning message from Shultz in London.