After I posted the message above, I recalled that I had a x1000 ND filter of dubious Chinese origin that I had never tried. With it, I could easily match an exposure time of 128s. So I prepared a test subject: color checker, its wrapping, some text and a piece of denim fabric.
One exposure (without filter) was taken at f/8, 1/10s. The other one (with filter) is taken at f/8, 128s. Daylight (overcast day, coming out of the window). Camera is H4D-50. Possibly, there is a slight defocus with the filter (I focus without the filter and then added it, it is too dark to focus with the filter on).
Post treatment: files open in focus, slight raise of exposure on the 1/10s to match the other (+0.3 ev), on both files white balance on the grey patch of the color checker.
The amusing part is that you'll have to find out which is which.
First, the complete scene, then a detail.