It would appear that the knob you have is meant for a smaller Joystick like the Parallax Joystick Module they sell at Radio Shack. The first picture shows the Esplora knob on the left next to the Parallax knob on the right-
The Parallax Joystick has a thinner, metal Joystick Rod so the hole is much smaller-
There must be a batch of them then... I just bought one from a Radio Shack here in Orlando and it too has the knob with the small rectangular slit, instead of a rounded hole. Although, I suppose I could simply drill it out.