This is 1/2 of the bolt and cap setup that is used as the axle pin for the roller on the 2500 "Traditional" Pro models (the model with the black rollers). It is the cap portion.

Length: 0.0000 in
Width: 0.0000 in
Height: 0.0000 in
Weight: 0.100 oz
Packaging: 0.0000 kg
Units in box: 1
Parts by Make/Model > Kahler Parts > 25xx Traditional - Fulcrum
Parts by Type > Saddles > Saddle Hardware