Ian, first point of interest to determine a kabuto is the kabuto itself. The mei can only be a confirmation or another question.
The hachi in the Met was seen as a Haruta by Sasama. The details of the , yes, zunari confirm this, no matter what Sasama said.
This mei is very close to the mei on Mark’s kabuto. Compare Marks kabuto with the early edo Haruta, as there are Katsusada. Fujiwara and Yamato are names often found in Haruta mei, as you know. Honorary titles, marketing...whatever.
Am I sure? Never. But I am sure that I am close.
I respect your opinion of course, but what school do you think it is?