I was rather thinking of Lehmann-Haupt. who wrote the entry on the inside back cover. look at the b's.
I think that these additional marks used to correct stitching, or of the order of reading the pages considering incorrect stitching.
The figure shows the correct stitching QUIRE 9.
In this scheme, Quire label gets in the right place. And figures 68v2 - 68v1 - 67r1 form a logical sequence.