This is quite interesting!
The palimpsest idea is perhaps a little bit too adventurous. With the hundreds of people who have inspected the manuscript, among whom there were certainly several dozens of highly qualified manuscript experts, the chances that the MS is a palimpsest, which somehow escaped attention are vanishingly small.
Ink or paint transfer from other pages is our best bet, and one thing that stands out is that the dark blue paint has been particularly sticky onto other pages.
Now the bifolio comprising folios 67 and 68 has received a lot of attention, since it seems to have been bound incorrectly into the manuscript. This was discussed already more than a decade ago, by one of the Groves (I believe it was John, not Jon), Nick Pelling and others.
This is how it has been bound into the book:
However, possibly, it should have been bound through the fold between f67r2 and f67r1.
In the first case (the present one), if one folds the folio in completely, the opposing page would be f69r.
In the second case, it would be f67v2.
(This takes a bit of mental origami and I hope I got it right).
Now let's take a closer look at the smudges between 11 and 12 o'clock:
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There's a row of circles, and some vertical lines. I think that these match up quite well with blue paint that may be found on f69r:
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This tells us that, when the blue paint was still wet, the bifolio was bound into the MS in the way we see it now.
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Edit:
(first edit because the external image link did not work and had to be replaced by a local image)
Second: actually, I have already been reminded before by Alain Touwaide that the bifolios may have simply been stacked after having been written and painted, well before they were actually bound into a codex. We don't know if that was the case, but if it were, the 'mistake' was already made at the earliest stages of the MS creation. This would fit with the observation made by Paula Zyats that the pinholes seen in the fold between f67r1 and f67r2 are simply knife cuts, and have never actually had any thread go through them.