RFH's are not the exact stated dimensions in the length. The height stays constant at 56mm (5.6cm) image opening and rounds up to 6 cm for convenience, while the image length can vary and it too is rounded up for convenience.
Nominal sizes are here:
See the last post in this thread: http://www.graflex.org/helpboard/vie...ight=film+gate

My Graphic 23 has a 56mm x 74mm film gate and is an early production one, says Graphic 23 on the shell. I have not measured other makes of holders.
On my Super Speed I removed the focus panel and marked the frame lines for a Graphic 22 (6cm x 6cm) and Graphic 23 (6cm x 9 cm) with a #2 pencil on the ground side of the glass.

On a Graflex SLR a piece of card stock or poster board can be cut to fit the gg opening then the RFH film gate size removed from the center for a mask that can be installed and removed easily.
The view hood will have a clip center of the front edge that will release the base of the view hood for gg cleaning on most Graflex SLR's.