Ed Wenn has added a photo to the pool:
Good news. I realised today that I had bought a Polaroid 405 film holder - one that let's you use the 6XX film pack series in 5x4 format cameras. I tried it on my Graflex RB 5x4 (with the home-made back modification) and whoo-ee....damned if the 405 holder didn't fit just as snugly as the 545.
I popped out into the garden to test it and could only come up with this shot of a clothes peg (not much going on in the garden at present), but it proved the point at least. Nice!
Fuji FP-100B pack film.