The Mamiya Six (not ot be confused with Mamiya-6) is a range of medium format folding cameras, providing negatives 6x6cm in size and coupled rangefinder. Being coupled means that the rangefinder is "linked" to the lens - or, in this case, the film plane mask - so that when focusing in the viewfinder the picture is also focused. With the uncoupled type, first the distance is measured and then transferred manually to the lens by means of rotating its front element.
Amongst folding cameras, they are not the lightest, but once closed they are about 5cm thick. Is easy to handle, even without strap latches, but this is overriden with the ERC.
The focusing method is one of the most curious details of this camera: opposite to the vast majority of folding cameras, that focus by movements of the lens - either the block or only the front element - the Mamiya Six focus by moving the film forward or backward relative to the lens. A metal mask is inserted once the film is inserted supported by two tabs that move the film during focusing. This focusing can be achieved with a wheel located at the read part of the top cover. People used to 35mm tends to think that the wheel is for film advance.
Another feature of those cameras is that the advance is linked to the shutter, so that in the Automat models the shutter is cocked at the same time the film is advanced, and that (as opposite to the method used by the famous Kodak Retina) does not work if the camera is closed. In case film is advanced with the camera closed, the shutter must be manually cocked by means of the lever located in the upper part of the shutter housing (this same procedure for shutter cocking is to be used for double exposures).
The Mamiya Six was the first product of the factory, even before being able to manufacture their own glass. During the 50s, when most of the cameras from this family were made, Mamiya used glass made by Olympus and labeled D.Zuiko. That "D" does not refer to the initial of some designer or engineer, but rather is a code that defines the number of lens elements (D=4, E=5, F=6...). So, a D.Zuiko lens has 4 elements, and it follows Tessar formula. By the end of 50s, Mamiya had already begun to manufacture their own glass, and one can find Mamiya Six with Mamiya-Sekor, 4 elements lenses also following Tessar's design.
6 different Mamiya Six models were made (variations apart). The three first during and right after WW2. They mounted a waist level finder in addition to the direct viewfinder, so they are easily spotted because of the three windows in the top cover (in a front view, the one at the right is the WLF, while the other two are viewfinder and rangefinder. The Mamiya-Six III added automatic advance and double exposure prevention. Those units with NKS shutters fetch higher prices amongst collectors and users, while models with Zuiko lens and either Copal or Seikosha shutter reach more reasonable prices.
The fourth evolution meant the disappear of the waist level finder, and this fourth model was the only one to be made between 1948 and 1953, when the fifth version introduced added multi format capability by means of a 6x4.5 mask. The Six-V was also manufactured in a more affordable version, the Six-K without automatic film advance (so each frame is positioned using the ruby window in the back of the camera).
In 1955 the numbering system was re-made with the introduction of a second variation of the fourth model, the Mamiya Six-IVb, a new, sharp design with more straigth lines and easily identifiable because of the big rangefinder window in the front part of the top cover. There was also a K2 version, same as the Mamiya Six-K but with the new body design. Later evolutions include the IVs and P, with minor cosmetic changes.
By the end of the 50s interest in folding cameras was decreasing in favor of rigid camera designs for 35mm film (even the Retina acquired a rigid body!). Only two further modifications were made by Mamiya to this family, the Automat and the Automat 2, which added the linking between advance and shutter as mentioned.
As is usual in most cameras from this era including that system, the automatic film counter is the wakest point of the camera, and if broken is a hell of gears and springs to repair.