The Leica M3 is a 35 mm rangefinder camera by Ernst Leitz GmbH (now Leica Camera AG), introduced in 1954. The M3 introduced several features to the Leica, among them the combination of viewfinder and rangefinder in one bright window and a bayonet lens mount. It was the most successful model of the M series, with over 220,000 units sold by the time production of the M3 model ended in 1966. Below are some photos of a beautiful cutaway model of an original Leica M3 camera (around serial number 800000).
(Source: Leica Shop Vienna's eBay, via Neotype)