Hi Alan,
Beautiful patination on this one!!
As noted, I believe this to be a British officers short sabre of 18th c. and a remarkably similar iron hilt to your example is shown in "Swords and Blades of the American Revolution" (George C. Neumann, Stackpole, 1973, p.106, #134.S). This example of these 'four slot' hilts has the same pommel/capstan and gadrooned grip. These apparantly were favored by both British and American officers. It should be remembered that in this period, c.1775-1780, individuals all were essentially considered British.
The straight blade on yours is straight, and similar to those seen on similar 'short sabres' c.1760 (Neumann, op.cit. #133.5). In this period through the 1790's some straight bladed swords were termed sabres, and the term 'spadroon' was also applied. I think Paul can probably better elaborate on the application of these terms as his knowledge on military swords of this period is well known.
I find the striations at the front of the guard most interesting, as they may represent influences other than simply aesthetic decoration, and it would be interesting to seek others opinions. These may be stylized marks recalling the scallop shell guards of earlier English hangers, or possibly even similar striations seen on Spanish shellguard weapons of this period. From a more esoteric perspective, it should be noted that there are distinct possibilities of Masonic associations and symbolism, and naturally in these times, both Colonial and British officers, typically as gentry, often had such Masonic affiliation. Any ideas guys?
All best regards,
Jim