I found something interesting while playing with my setup.
With 6x8 back, there is a little bit of play when it's set on the revolving adapter. (when it is unsecured) It will slide just a little, about 1mm, along the track. I rotated the back to vertical position and released the back, then pushed the back UP, then secured. Because the top edge wasn't sharply defined, I knew the bottom was the problem. I then took some test shots.
With this, in vertical orientation, all edges are sharply defined. In horizontal position, just a very little on both bottom corners are vignetted. Just VERY VERY little.
So there is some manufacturing tolerance in play here.
I actually ordered a second body to compare and keep as a second body. Unfortunately, this body came damaged so I neither could use it or test with it. (it went back). As far as dimension goes, it was identical to mine, except the "play" was even bigger. This confirms manufacturing tolerances is part of the issue.
I'm not sure if this helps anyone.
I heard from a third party, with some lens and distances, more or less vignetting can be experienced. I'm trying to get a documentation on this.