Actually, it is 6x6 and it is 6x4.5 (with the optional mask components). What it isn't is a "true" 6x4.5, in that it still consumes the same amount of film as a 6x6. So it is really a 6x6 image masked in camera to 6x4.5.
Originally Posted by mrcallow
I find that option useless, as I stated before. I own the 6MF system, and find it to be a *great* camera, due to its lenses, its quiet shutter and its portability. But a great 6x4.5 camera it isn't.
Many folks opt for the earlier version the "new" Mamiya 6 (not MF), because the MF introduces extra lines in the viewfinder to represent the 6x4.5 and 35mm mask points. It is supposed to help you frame the shot when those masks are installed.
Personally, I like the extra lines, bacause sometimes they are useful references mid-frame to help keep things visually level - especially when using the 50mm (wide angle) lens. But I've never had a need for the masks.
I have no idea what motivated Mamiya to introduce this option. Looks like they accidentally staffed a bozo in the marketing department. It happens...