[Jim McDougall]

This is a total anomaly, and in risking a totally speculative assessment, the thing I think of is a Spanish 'pappenheimer' type weapon with saber blade which seems to be perhaps Styrian or S.German (by the fullering, decoration). This would be of course mid 17th c. contemporary with Pappenheimers as well as Spanish cup hilts, shell guards (margarite).

Perhaps this might have some connection to Spanish Netherlands ? (1556-1714).

The punzone on the tang does not appear in Palomares*, but is Spanish in manner. In Kinman ("European Makers of Edged Weapons, Their Marks", 2015), p.142, a similar mark with C under crown in dotted cartouche is shown as Zamorane el Toledano, noted pre-1700.

This mark is the crowned C, but with what seems an S (?) enclosed. This could be a spurious 'Toledo' mark as used of course in Germany (Solingen and Munich) and this blade seems 18th c. .

In Wallace Collection (Mann, 1962) there are numerous cases of older hilts mounted with later blades, so these kinds of pairings are far from unknown.
I have a 1750-60s British basket hilt which was infantry used, but in 1783 they stopped carrying swords. The swords went into stores.
At some point later my example was refitted from the straight blade to a M1788 light cavalry saber blade....I have seen two others with this modification. It is hard to say what would prompt these changes, but personal favor or heirloom hilt to a more contemporary blade would seem plausible.

* corrected, the mark does appear in Palomares #94