This is the Fuji 6 x 4.5cm (645) format mask. A matching 645 focusing screen is available.
Note! A mask is NOT required for the 6 x 8cm format - These masks fit ONLY the GXIII back (B&H #FUFBGX3)
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