A plug that keeps the upper pieces of your pole from falling out of the bottom. Just put this plug inside of the cap with the bump facing down, and screw the cap back on. You may need to take out a rubber o ring from inside of the cap if you have one.
The outer edge is 23mm in diameter, and the part that sticks up is 19mm in diameter.
The last image is a picture of the pole this is on.
The author marked this model as their own original creation.