The Zeiss Contax line was the only major challenger to Leica's preeminence in high-end 35 mm rangefinder cameras. Before WW2, the model II and III were the hot ones. The III was the same as the II except with the addition of a selenium exposure meter built into the top of the camera.

After WW2, the heavily redesigned IIa and IIIa replaced them.

When I was a camera repairman, Contaces were a particular specialty of mine.

These cameras featured a vertical-travel metal focal-plane shutter, similar to the top of a roll-top desk. They also had a very wide-base, highly accurate rangefinder. The Contax rangefinder/viewfinder was superior to that on the screw-mount Leicas, but not as good as the M Series models.

The Zeiss lenses were made for maximum speed and contrast, while the Leitz lenses had the reputation of being ever-so-slightly sharper.

The normal lens on the Contax is a 50 mm f1.5 Sonnar, but a range of lenses were available for it. I used to own a 21 mm Biogon, a killer 35 mm Biogon, 85 mm Sonnar and 135 mm Sonnar. I also had a Kilfit reflex housing with a 150 mm preset lens. The Contax had no provision for single-window viewing with anything but the normal lens, though there was a clip-on mask that made it sork after a fashion with the 85 and 135, though the framing was somewhat approximate. A common accessory for these cameras was a separate viewfinder that went into the accessory shoe, with a revolving turret for different focal lengths, and a manual parallax adjuster cam to tilt the finder to the appropriate angle. Generally, quite cumbersome, so these cameras are not all that useful with the long lenses.

Nikon rangefinder cameras copied the two-part Contax lens mount, so when I had my Nikon SP I frequently used the Contax lenses on it.

These cameras were beautifully made, exquisiter German craftsmanship throughout. I've owned a number of them over the years, and I still own this one.