Death at an Old Mansion should not be part of this collection. It may be a Kousuke Kindachi film, but it was made by different people and not by Kon Ichikawa starring Koji Ishizaka as Kousuke. It is unrelated to the other 6 films in the series.

https://ja.wikipedia.org/wiki/%E7%9F%B3%E5%9D%82%E6%B5%A9%E4%BA%8C%E3%81%AE%E9%87%91%E7%94%B0%E4%B8%80%E8%80%95%E5%8A%A9%E3%82%B7%E3%83%AA%E3%83%BC%E3%82%BA#%E4%BD%9C%E5%93%81%E4%B8%80%E8%A6%A7