These bowls are authenticated by BuddhistArticfacts.com and are guaranteed to be 100+ years old.
Ultabati singing bowls similar to Jambati Singing bowl style and come in similarly large sizes over 7″.
Their distinguishing feature is that the side of the bowl is curved in under the rim. These bowls are not very common which makes them difficult to find with better sound quality.
Ultabati bowls often have prominent hammer marks. They can come darkened, even black, on the outside and bright in the interior.
Etching lines are similar to Jambati bowls but much less commons.