The origin of Fabrizio Salani's "AG" monagram

[Koen G]

Or perhaps a cutout from somewhere else attached to the parchment makes sense, like an old wax seal collection

I like this idea, it would allow for a number of apparently conflicting observations to coexist:

* Several indications of modern inks.
* Wax seal has signs of aging, with potentially 19th century design.
* Fabrizio had the impression the accompanying documents had been collected for their wax seals.
* Cardboard made of flax/rags: wood pulp became the undisputed standard material for paper making ca. 1900. This means the cardboard is either pre-1900 or made in a "traditional" fashion.
* Animal-based (protein) glues are still used for a number of application, with bookbinding as a notable example.