https://www.militaria.co.za/nmb/topic/9481-ubu-ha/?do=findComment&comment=96062
Actually, ubu-ba refers to the blunt, as yet unsharpened part of the blade that usually is seen from the ha-machi forward for a few inches on blades that have only seen a few polishes. With more polishes, this "virgin" edge is removed until the entire blade is sharp. Thus, a blade that exhibits ubu-ba is in nearly as made condition...
It is usually seen on shinsakuto, and often on WWII era gendaito. It is usually not seen on showa-to or machine made blades. It is rare to see on shinshinto. I have seen a few shinto blades with ubu-ba but that is very very unusual....
https://www.militaria.co.za/nmb/topic/9481-ubu-ha/?do=findComment&comment=96062