As a beginner, like Herbert (and humble also), I do wonder the importance of this controversy as to when thumb-out technique was developed. I would have to guess that some of the paintings of the period take a certain liberty with the details (while others may be professionally exact). And I have serious doubts as to the strict observance of "rules of play" by talented musicians of the time. I remember years ago, when I was teaching skiing, the Ski School Director and I were fooling around on the mountain. Later in the bar (which he owned) a young lady came over and asked him why he taught her to keep her weight on the downhill ski and forward when he and I had been breaking all the "rules". He said "Do you remember how awkward you were on your first pair of high heels? I've been on skis all my life and they are as comfortable to me as bedroom slippers. I can break the "rules", you have to learn the right way".
I have the suspicion that the great players played more freely than they taught. Best, Jon