I think this attribution is pretty much spot on, and this curious mark seems to ring so familiar, yet despite its simplicity it is a tough one to find.
In Wallace Coll. (Mann, 1962, p.371, A728; A729) there are two German 'rondel' daggers described as German first half 16th c.
The marks appearing on both have heavier versions of this dual arcs joined by vertical line mark, stated a makers mark but no identification given. There are apparently others with the same mark in various museum holdings.
The closest thing to this simple device found among makers marks would be that of Johannes Kohl the Elder (1580-1620), which has the arcs, vertical line but with a transverse line bisecting the vertical one (Kinman, p.47). This is one of this family marks, but seems to fall into the probable period of this left hand dagger. These, while falling out of use elsewhere, were becoming more well known en suite with rapiers in Spain and Italy.
Until more precise identification of the mark is found, it seems plausible it may be a variation of these German marks of unidentified maker in mid 16th c. , perhaps adopted in variation by Kohl (he later used a majuscule K or a crucifix.
On the other hand, uh, the 'left'
this could well be an Italian version using that mark from the repertoire of German marks sometimes used in Italy, and produced first half of 17th c. This does seem more compelling given the baroque manner of the motif.