Early stubtail by Roberts:
Attachment 298106Attachment 298107
The handle was obviously a later addition, but the blade is probably late 18th-very early 19th century. I'm not sure which: on one hand the shoulder is wrong for the late 1700's, on the other - dip-at-toe, thin oval-sectioned tang and very thin stubtail. Maybe it was a regrind, and maybe it's from around 1810.