An "old maid's tooth"? Interesting name for a "fuller". But then again, you call "C-Clamps", "G-Clamps" so there's probably a ton of different names for the exact same tool we use over here.
There are a wide variety of fullers, but basically a fuller is positive forming tool (as opposed to a swage which is a negative forming tool). All are "struck tools", not "striking tools", some are used in pairs either hinged, sprung, or as a hardie & handled set.
Hinged fuller - In this case the fuller is being used to isolate blade material from socket material of a spear head.
Top fuller (handled)
Bottom fuller (hardie)