Well, I found one of the bad film holders.
Shot on a piece of black foam board at the minimum aperture of my 125mm lens. The lens IS sharp.
This is a negative, scanned wet because I was ticked at it. That is where all the weirdness comes from. The other exposure I took, which was the shorter of the two, was completely dark. This one is exposed pretty correctly, meaning I did end up calculating bellows factor properly.
I'm wanting to get some Foma 100 film to continue playing around with this lens. I think that outside, at closer to infinity, things could get interesting.