Hmmm. To be 6x24, with 224 degrees coverage, it would have to have a center core a bit more than 8 cm diameter. It didn't look that big to me, compared to the 120 spools shown in the interior picture. 6x17 would have a core a bit over 6 cm, which is close to what this looks like, while 6x12 would have about a 4.5 cm core diameter. I'm thinking it's 6x17, now.

Loading in the dark is a major pain in the arse with a common 120 camera (I've done it once, after realizing I'd missed frame 1; I unloaded and reloaded in my changing bag, and even with the film already caught on the takeup it was quite a chore), I wouldn't expect many folks to be very happy spending that kind of money on a custom made, hand finished camera and then finding out they have to load it in a changing bag. I've heard of people pasting extra leader onto the 120 backing paper to load 6x17, and I suppose that would work on 6x24 just as easily, but there comes a point at which the extra leader makes the roll bigger than the spool flange and you get edge fogging...