I believe both parts belong together.
Typical 1728s as the ones shown here are rather post 1760.
From the end of XVII century there were a series of experimental designs.
We have to think that
boca-de-caballo swords in aspect evolve from the two shells Brescian hilts, and eventually from the Pappenheimer.
http://www.vikingsword.com/vb/showthread.php?t=27384
But as a construction, because of the use of screws, they are related to cup hilts. And there are cup hilts with screws but shaped as Brescian hilts.
Initially boca-de-caballo swords had only two screws, but this made the two shells prone to colapse and break at the middle of the line between the two screws.
One of the solutions was to add a second set of screws at the sides with a reinforcement piece (later was welded to the pass de ane). I call these rhomboidal 4 screws hilts.
The blade seems to come from the same Solingen workshop as the "Enrique Coel" blades, just with no lettering.
I would not wonder too much on the Martinique subject. We know cavalry swords were sometimes embarked in the Spanish fleet, and the Royal Armouries at Leeds has a bilbo captured at Trafalgar. Any shipwreck could carry the sword to the island where most probably was used as a machete.
That the square has more room than needed for the ricasso is not new, as the structure is there to hold the shells not the blade.