A hypothetical before I attempt looking for something like this. If there is a passage in the manuscript with some obvious defect of the quill, say, a stray hair that leaves an occasional thin line offset from the main stroke, would you consider this evidence for MIO and against MRT if this defect follows the same distribution of ink shade variation as the main text?
I'm trying to think of anything that could in principle falsify MRT, otherwise for me it is kind of stuck in "not even wrong" territory.
I'm trying to think of anything that could in principle falsify MRT, otherwise for me it is kind of stuck in "not even wrong" territory.

