As Dutch said, the large handle under the duplex air gauges is the horn handle. You had mentioned the handle and button on the console, to the left of the Master Controller handle. The button is a console buzzer button. The Atari joystick-looking handle is the acknowledge handle.
It will perform several functions, including the following two acknowledgement functions:
It acknowledges that a more restrictive ATC code has been received.
It also acknowledges,satisfies, and resets the Alerter timer circuitry, as outlined in above posts.
There are two distinctly different sounds for code change and alerter.
An ATC downcode nets a fairly loud "beepbeepbeepbeep", while an alerter tone is a continuous "dee-doo, dee-doo", gradually increasing in volume.