[Design of Marci Letter]

[asteckley]

I've long wondered what the basis was for this "19th c. rebinding by the Jesuits", supposedly producing the current cover, and why the cover is said to be "18th or 19th century in origin". But I have never been able to find a physical, forensic reason for it.
…
Perhaps someone knows of such physical proof this cover could not have been the one seen and handled by Marci…

It is possible to get some additional evidence for or against that claim by looking at light table pictures of the  back endleaf.

Due to improvements in the process of making laid paper, later era paper showed less shadowing along the vertical chain lines. The Marci Letter shows such shadowing but you see it only on light table images.
The endleaf may or may not show chain shadows (or watermarks).

Unfortunately, it would be very difficult to take such imagery on the endleaf — the manuscript is too fragile and delicate to risk it.

The issue would not affect the core hypothesis though; the evidence that the letter was bound (or “tipped-in”) is stronger than evidence that the manuscript could not have been rebound without preserving the binding-in of the letter.