Complex sentence: Wallis retired after making the film "Rooster Cogburn" (1975) at Universal, a sequel to "True Grit" (1969), which Wallis had produced at Paramount.
Split into simpler sentences: Wallis retired after making the film "Rooster Cogburn" at Universal in 1975. This was a sequel to the original "True Grit", which Wallis had produced at Paramount.