Sorry for the slow reply. I don't check in here as much as I used to. The last time I attempted to correct some blatant misinformation, I was told by the administrator to block the other person and ignore their comments. He also admitted he nothing about Japanese blades.
That did not set well with me. I am a champion for correcting misinformation in a sea of misinformation regarding Nihonto. I will not ignore BS that creates confusion. If this site prefers to allow BS opposed to facts that is their prerogative.
In regards to this sword, this is not a group I know much about. There many Yoshisada which spanned across the ages and schools. Ishihara smiths which signed using Seki were from the Mino area. A quick look in the books only revealed one Ishihara Yoshisada using this combination of characters from Gifu who worked in the Showa period (1926-1989).
Additionally, the photos provided are not great for gaining information. There are more photos of the fittings than the sword and to evaluate the sword you need photos of the sword. The photos do not show the entire tang and I would be surprised if it didn't have a seki stamp.