I concur with M.Eley.
The therm "Nata" has a quiet large meaning as regards shape of blade and is sometimes misused for a type of Tanto (Kubikiri) that was used to severe the heads of the enemy for later display, that has a shape vaguely similar to your blade, that IMHO is still a tool and not a weapon.
The following first is called Nata, the second I'm not so sure...
