It's a phenolic faceplate with a knob on it that was turned in order to sound a bell for the Western Union guy. My guess as to the instruction to turn it 3 times is that it was installed in a building that had more than 1 place to pick up packages from.
Maybe the mailroom sounded once, the secretarial pool sounded twice and the boss's office was thrice so that Western Union guy knew where to pick up the outgoing telegram from.
