Although that is the sewing line now, if you notice the other side, it is repaired but still dirtier, as though it was once the fold when it was just a quire. Look at the 2004 scans to see the damage and dirt before it was repaired. I think that was the case before it was sewn in, and that they chose to sew the side they did to avoid the damage, even though it changes the page ordering once you fold it. Not sure it would ever have been sewn that way but was probably the actual outer fold when it was just a loose quire, and was damaged from the friction of being on the outside, and possibly because it was used to hold other quires in the folds before they were bound, ie friction from the inside as well as perhaps some stretching. If you treat it that way, the page ordering starts to make more sense, the back page with the TO design on it, that now has the quire mark, becomes the front, and the quire opens to text only pages (f85r1 and f86v6 , with another text only page at the back (f86v5). Then surprise, opening the inner folded page reveals the two hidden drawings.
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some of the damage from the outside and how dirty the outer fold is, also supporting the idea that this was the original outer fold of the quire.