What were the earliest instances of Kamal Haasan (then, Kamalahasan) being involved in the writing of a film.

1. Unarchigal (1976) -- He co-wrote this film with R. C. Sakthi, the director of the film. He also rewrote the film for its Malayalam version.

2. Meendum Kokila (1981) -- The story is credited to Hasan Brothers (Kamalahasan and Chandrahasan) -- Ananthu wrote the screenplay (incl. dialogue). In a way, this served as a precursor to their first home production (Hasan Brothers), Raaja Paarvai -- written by Kamalahasan -- that came later in the same year.
[Bonus trivia: The title, by the by, is a take-off on Balu Mahendra's maiden film Kokila (1977; Kannada), which starred Shobha and Kamalahasan.]

The protagonist of K.B.'s Unnal Mudiyum Thambi is named after

Udhayamurthy (Kamal Haasan), the protagonist of the film, was named after M. S. Udhayamurthy, founder of Makkal Sakthi Eiyakkam.