I shoot tons of whole plate every year. My reason is off the charts from anything suggested so far. I buy the Agfa Aviphot aerial recon mapping film in 240mm wide by 250 foot rolls. So I roll out the film to the 6 1/2" width, make a cut, then trim the 9 1/2 down to 8 1/2 and load my holders. All of my holders are Toyo at this point. They used the old Kodak standard measurements for their holders. Each shot costs me about $.45 cents and the film is wonderful. It's ood when I buy it. A guy in India has sold a lot of it in feebay. BTW, off the top of my head your flange looks like it could be American Ilex size 5. I can get an exact measurement tomorrow if you like. Late here in western USA. Do the Rittreck film holders you've shown fit properly? The light trap was bigger than Kodak's and they didn't fit well in my old Kodak full plate UK back. Close but not light tight. If the lens flange is Ilex 5 a 12" Ilex Paragon in barrel is wonderfully under rated and cheap. You could be making very fine contact prints very quickly. I like full plate centered on 11X14 paper with white rebate.
