5 pack Brass Potentiometer Adapter Sleeves.

6mm inside diameter and 1/4 inch (6.35mm) outside diameter x 3/8 inch tall.

These are used to install set screw knobs that were designed for 1/4 inch solid shaft pots on split shaft pots (both fine and coarse splined) and metric 6mm solid shaft pots.

These will increase the diameter of the pot's shaft so the knob will sit centered on the shaft.