What's this?

Pencil cap?

Answer: A device for dialing a number on a rotary telephone. Today’s youth may not have seen rotary telephones, but older people probably used these “ancient” devices by modern standards. This retro device was worn on the end of a pen or pencil and greatly facilitated the task for those who, for some reason, preferred not to dial the number with their fingers (for example, girls with freshly painted nails).