It's the lens body itself. The part immediately after the screw thread where it goes out wide.
The thing I noticed is that it's also quite long compared to my meopta anaret-s. I'm wondering if this means it needs to go currespondingly further in the up direction to focus. This might make it even harder. It's really a matter of millimeters from getting focus with a flat board. If it was a touch shorter, I wonder if it would work.
Oh well. It doesn't!