This is a gassaku — a collaborative composition or collective work in which more than one artist is responsible for the design. It is not clear what Baien, who designed the center sheet, intended when he appended sha to his signature. The term had various meanings, includinig "copied by," "drawn by," or "sketched by." Perhaps he was declaring that his drawing of Kichisaburô was done in the manner of Hokushû, who was certainly the senior artist on the project.
Although gassaku appear from time to time, they are uncommon, especially from the earlier period of Osaka printmaking, as here. When they were published in Kamigata (Osaka-Kyoto region), they typically involved a master and his students. There were also shared projects involving artists of equal status (some scholars argue that the term gassaku should apply only to collaborative works by artists with similar standing).
A very good example of a collaborative work from the second period of Osaka printmaking.