A call box was mounted on the wall of a business office. When the businessman wanted a Western Union (or Postal Telegraph) boy to come pick up a message to be sent by telegram, he'd turn and release the crank on his call box.
This would generate a series of pulses (usually a two or three digit code). At the telegraph office, many selectors (Gill selectors) were connected to the wire.
A selector is like a relay with a gear escapement matched to a certain series of pulses.
The pulses received over the wire from the office call box would trip a specific Gill selector, closing a set of contacts and lighting a lamp or otherwise indicating the origin of the call.
The telegram boy would them know which office to go to.
Metal oval deep blue porcelain finished cover, black knob, white porcelain base.
The base is about 3-1/2 x 6 inches. The mechanism was made by Hammarlund.
The code wheel is 1-3-1. Inspection stamp on bottom.