^ I'm not doubting you
but all other evidence seems to point to different results than you've been getting
the camerapedia states
The lens system incorporates a breechlock mount unique to the Mamiya Press, and each lens is mounted in a Seikosha leaf shutter. Lenses of fixed focal lengths from 50mm to 250mm were produced, all of which covered up to 2x3 film sheets, while the 127mm and 75mm lenses offered greater coverage necessary for the 2 7/8 x 3 3/4 inch images produced by the Polaroid backs. "
although camerapedia may not be the definitive authority.
Then if you go to this guys page
and scroll down halfway he states:
"Anyway, on to the Universal. The most obvious advantage the Universal has over the 600SE is the lens selection. This comes with a caveat, though...most of these lenses were designed for 120 film. The area of Polaroid film is larger than the largest 120 back available (6x9). This means that some of these lenses have falloff beyond the normal area for 120. That's vignette to you and me. Now I, personally, don't take this as a negative. But, depending in the lens, it's kind of like having an expensive, super-sharp toy camera! Here is an illustration of the different exposure sizes with the 100mm lens for the Mamiya Universal."
and shows example jpgs of polaroids with corner falloff/vignette with a 100mm 3.5
I don't own a Polaroid holder and the vignette wouldn't be a deal killer for me anyway but I'd like to know if sharp corners are possible.