When did Doc Adams leave the Gunsmoke show?

The late Milburn Stone (July 5, 1904 - June 12, 1980) who played in the role of "Doc Galen Adams" made his last appearance in the "Gunsmoke" series in the episode "The Sharecroppers" (31 March 1975) . I remember that he was absent from the series due to having heart surgery but I couldn't find which episodes in which he was absent then returned .

Actor Pat Hingle , playing in the role of "Dr. John Chapman" , made his appearance in 1971 ( Gunsmoke: Season 17, Episode 11 - "Drago" [22 Nov. 1971] ) which was about the same time the late Milburn Stone was temporarily replaced so I'm guessing it was in 1971 .