There are roll film holders available apart from the Rada there's also the Rollex - I have 2 in 9x12, and I've seen them on Ebay to for 6.5x9 cameras. I have some adverts for the Zeiss cameras back in Turkey and by the mid 1930's it was quite common for that format camera to be sold with a roll film back by some dealers. I'll check on my return.

Later post War Rollex backs are made by Linhof and are a different fit.