Martin, after that thrilling romp through deductive reasoning (wearing my deer stalker hat and pipe) which really did seem logical,
look what I found in my notes and archived images!!!
It is maddeningly uncited, but listed as a 19th century Ottoman Turkish military sword.
So the question remains, where did this finger stall feature come in? and since it is so prevalent on Italian swords, what prompted the influence?
Note this Ottoman sword has the similar protruding quillon.