The New-York Historical Society’s collection includes this archeological piece:

Iron. Overall: 2 1/4 x 4 5/8 in. ( 5.7 x 11.7 cm ). Object # INV.5924.47. Collection of the New-York Historical Society

According to the Society:

This sword guard was excavated by Reginald P. Bolton, William L. Calver, and others prior to the formation of the Field Exploration Committee in 1918, at the British camp on the Dyckman farm, between Seaman and Payson Avenues and 204th and Academy Streets in Washington Heights. The sword belonged to a British officer, grenadier, or sergeant, or to a Hessian officer.

While the Society identifies this as a sword guard, it’s more likely a remnant of a fencing foil. Compare it with this extant foil which, apart from having a deeper bell guard, is very similar to the N-YHS piece:

C. 1770. Earliest known extant English foil, made by Samuel Harvey of Birmingham. From the collection of the National Fencing Museum.

Thanks to Erik Goldstein, weapons scholar and Curator of Mechanical Arts & Numismatics at Colonial Williamsburg, for bringing the guard to my attention and suggesting it was for a foil.