Posted by print is by Yoshitor (172.142.100.195) on January 16, 2006 at 13:43:34:
In Reply to: Re: Hosai Baido posted by david on January 14, 2006 at 12:12:25:
I found another example of this triptych in the NDL database, the set is actually by NAGASHIMA MOUSAI, also known as Utagawa Yoshitora. (The signature is MOUSAI not HOSAI)
So I answered my own question - if it had been by 'Hosai Baido' then it would be considered a later derivative design.
But this Yoshitora design was around the same time as Yoshitoshi's so they could both be conceptually derived from some other earlier print, if they are related it's not clear whose is derived from whom.