John Carter (Willis Clark) was at the time this episode aired a regular cast member on Henry Fonda's sitcom The Smith Family (1971).
Fifteen-year-old Billy Clark was played by 19-year-old actor Billy Bowles. Bowles had the previous year played a college student in R.P.M. (1970). His youthful appearance continued to net him guest spots playing a high schooler on Room 222 (1969).
That name sounds familiar. Don Eitner, the doctor who made a house call to treat Billy, later played a character named Pete Cockrum on Make Me a Perfect Murder (1978).