Fuji GX680III 6X4.5 Format Mask for Film Back
This is the Fuji 6 x 4.5cm (645) format mask. A matching 645 focusing screen is available.
The Fuji GX680III series cameras offer convenient multi-format capabilities; To do this, they use a system of masks which install in the front of the film back
These masks "communicate" with the back and alter the winding, spacing and number of frames