Pack of 6 ceramic 1/16" thrust balls.
1/16 Silicon Nitride (Si3N4) ceramic balls are 30% lighter than steel and 40% stronger. Each ball is over 1000X more round than the best steel bearing of any type, grade or benchmark. In other words your diff will be smoother and last longer due to the balls not deforming and causing havok to your rings.
Check your instruction manual for the correct size for your kit.