Thanks for the link to Legacy Arts. I have tried Benson but he is busy. Hall seems to have dropped off the face of the earth. I see references to him but I can't find any contact information. Jimmy Hibiachi in San Francisco also seems to have gone missing. I have a phone number for him but no one ever answers. I must have seen Tenold's website but forgot it. I will contact him.
I have searched far and wide for swords with a sosho mei and the oldest one, I could find was
mid 17th century, consistent with your observation. "According to Yoshida, Shinkai Kunisada started daimei for his father in the second year of Shoho
(1645) with a sosho mei ("grass" or running/cursive style mei) ( Fig.4, 5) . Yoshida said that the reason for starting with a sosho mei may be as follows: Since this daimei was still unofficial, Oya Kunisada did not want Shinkai Kunisada to put the same kaisho-mei (normal or "block letter" style mei), written by an obviously different hand, on the daimei products.
Because sosho mei was new, nobody would immediately notice that this was daimei. This may be a reasonable guess".
More on sosho mei origins:
https://www.touken.or.jp/Portals/0/pdf/english/tobi_translation/687_NBTHK_April_2014.pdf
Now look what you have done. One more rabbit hole to plunge down. It looks like the plot has taken an interesting turn.