LBJ

A horn phone for a rancher president.

Initials of Lyndon Baines Johnson, 36th President of the United States. Johnson served from 1963 to 1969, covering the period of the majority of Maxwell Smart's recorded cases [Wikipedia].

At first only called "Him" [Episode #9: "Satan Place"], references to his ranch [Episode #33: "A Spy for a Spy"], grandson [Episode #81: "Operation Ridiculous"] and baby powder [Episode #76: "The Little Black Book, Part 1"] leave no doubt as to his identity.

This is confirmed in 1968, when, as part of a plot to gain access to CONTROL's files, KAOS agents tap into their hotline to the White House and, imitating the President's voice, promote Maxwell Smart to the position of Chief. At one stage, believing that he has been conversing with the real president, Max refers to him as "L", short for "LBJ", claiming that they are "on a first name basis now" [Episode #84: "The Hot Line"].

Johnson was succeeded by Richard Nixon.