Necker's Knob Meaning

(idiomatic, slang, dated) A knob attached to the steering wheel of an automobile, especially before the widespread availability of power steering, helping the driver steer with one arm and leaving the other arm free to provide romantic attention to a companion.

Example: 1963, Herbert Gold, Salt: A Novel, Dial Press, p. 246:
  You remember the necker's knob on your high school jalopy?