It looks like a LOT OF INK
has been spent in the discussion of this mark, both in origin as in purpose ... as well as in terminology.
Origin is consensually accepted as that from the Veneto area, while its purpose is 'arguably' to identify an arsenal. While in one hand Javier Ramos (for the case) textually says "I would have to check if my memory fails again but I remember vaguely that the "marca a mosca" (fly mark) is not an armourer mark but an arsenal mark (Venice)",
On the other hand, the fact that the mark appears either single or in trios, does denote that, for an (one) arsenal mark, is somehow inconsistent. Also interesting that there seems to be an intention to place the central of these (triple) marks in a different position. As for the mark naming, beyond the english connotation "twigs", i wonder why some Italian sources call it MOSCA (fly), and Boccia & Coelho, who are not ignaros in these things, prefer to call it FERRI DI MULINO