Upon re-reading the Covenant 150 submission for ArM4 I noticed something that I had missed entirely previously... Magus Talpa of Bjornaer writes in the summer of 1228 a Muto summa of L50/Q30!
Now, I'm not concerned with the quality of the book because this, in the 4th edition, is governed primarily by an ability (scribe) that can be raised to a ridiculous degree, in this case by a charm (+20) made by another Magus. I do question, however, how is he able to write a book of that level considering his Muto score is "only" 75 without his affinity or 80 with this included. I thought that the maximum quality for a Summae was Art level / 2, as is in ArM5. And indeed it is (p.188 ArM4).
Under the "remarks" text for Talpa, his creator discusses how a specialist was able to reach level 75, hence Summae of level 50 will exist for most Arts in the Order. That number is exactly 2 / 3 of the total, and I don't think it is a coincidence unless it is a mistake or there is a detail that I have overlooked. Is there a relevant virtue in ArM4 that changes the maximum level a Summae can be written or a way to add his scribe (25) to the total of his art (75+25 / 2) for that purpose? Perhaps something in the Wizard's Grimoire?