PS: This panel has 108 cells, arranged 6 by 18. Looking at the "bus" conductors on the short edges of the panel, it seems that 2 cells are in parallel along one short edge, and the other 4 in the same row of 6 are in parallel. On the opposite edge, it's 4 and 2 (swapped). Is it a good guess that it's two parallel strings of cells, going down the length of the panel, then back, then down again? That would be 54 cells in series for a total voltage of about 32 (if each is 0.6V), which fits the open-circuit voltage claimed for this panel.
If that's the case, and given that it's supposed to have only two bypass diodes, would it be one across a smaller secton, 1/3 of the cells, and the other across a larger section, the other 2/3? If that's the case, then shading 1/3 of the panel, i.e. 2 rows of cells along a long edge, would reduce the power by either 1/3 or 2/3, depending on whether the shaded 1/3 is part of the smaller or larger section?
I might do some shading experiments, but need some theory to guide them...