Welcome to the antiquities department.
Hobbed bolts are used in Wade's extruders, which are geared extruders, such as all the extruders seen in this post: viewtopic.php?f=16&t=3484
Wade's extruders - geared extruders - were pretty much necessary when plastic filament was all "3mm" in diameter. Then they came out with 1.75mm filament, which didn't require as much torque and so they started making "Direct Drive" extruders without the gear-down ratio between the motor and the driving point. So all the new Printrbots come with Direct Drive. Several of us old-timers prefer the 3mm filament and Wade's extruders. But I can understand how it would be confusing to someone who never saw, or heard of, a Wade's extruder.
That said, you can buy the hobbed "nuts" for direct drive from Arcol
, called "Mini Hyena."