Origin of the Shield Shape in the Solar Folio of the f85-86 Foldout

[Dobri]

Indeed, the point here is that four centuries later, in the early 15th century, the unknown scribes likely used elements of the Beneventan script to create the cipher.

Very unlikely. The U is not specifically Beneventan. The only similarity is the "re" that looks almost like k but the r is lower.

The Beneventan script was still used in isolated locations during the 15th century.
Also, the shape of the Beneventan letter 't' (with bow) could possibly be related to the creation of the bench and the benched gallows.