Several of us have been talking about a possible explanation for M-arcs. The responsible crystals are twinned trapezoidal plates that have what might be called restricted Lowitz orientations. The crystals perform rotational oscillations about a horizontal axis, with the equilibrium orientation having the vee opening directly upward, as in the left hand crystal.
All of us are more or less skeptical about this explanation, but we think it deserves testing. The model predicts a strong lower 20° arc. We are asking that, if you are lucky enough to see an M-arc, then please put yourself in an elevated position and look for the lower 20° arc. There are other observing suggestions as well, but I think I will leave them for the blog.