The gem, which belonged to a British amateur photographer who died last year, is one of four Luxus Leica IIs models ever made.
It bears the serial number ‘88840′ and the whereabouts of the other three models in the series is unknown, said Bonhams ahead of the sale.
Auctioneers had expected it to fetch between £480,000-£720,000 and media reports suggested it may break the £1.7m record set last year by another Leica camera.
However, the Leica IIs bowed out for HK$4.8m (£386,981 including buyer’s premium).
When shown to experts on the BBC TV’s Antiques Roadshow in 2001 the vintage model was valued at just £5,000.
The camera comes with a 50mm f/3.5 Elmar lens and a ‘crocodile’ camera case.
The owner had acquired the Leica soon after World War II.
Last year, a different Leica camera, an O-Series model, sold for a record €2,160,000 at the Westlicht auction in Vienna, Austria.