Those descendants continued to scrape by through working in agriculture, and by the time Seiichi was born, his family household was still making a living by running a business from the house in areas such as sericulture.

After graduating from university, he got job at a legal organisation and before long he transitioned over to working at a government cabinet research bureau.

[4][5] In 1967, Katsumata criticised Prime Minister Eisaku Satō and the government while the latter were in discussion with the United States concerning the possible reversion of Okinawa to Japanese authority.

[6] Katsumata was known within the party as a theorist, and assisted the formulation of the "Road to Establish Socialism in Japan" plan while serving as the Secretary General of the Socialist Theory Committee.

In the same year that he retired, he became the first Socialist Party chairman to be awarded the Grand Cordon of the Order of the Rising Sun, First Class.

At the National Diet Library's Constitutional Government Reference Room, stenographic recordings of his talks are open to the public after they were left behind.

Since the work of one of the postwar period's most visible leftists was heavily based around theory, his influence through mass media was quite weak.

[7] In the context of socialist theory, he was granted an honorary PhD from the East German Martin Luther University of Halle-Wittenberg.