This telephone attachment slips under the mouthpiece of a candlestick or wooden wall phone.  The hole on this one has been worn through, but it stays on the phone if the edge is screwed down under the mouthpiece and then appears correctly. A well-worn piece of quite rare 1910's-'20s ephemera. These were popular when toll calls had an initial period of 3 minutes only, and were not inexpensive.  The attachment advertises the Community Oil Company in Boone, Iowa.'