Shimano CS-7800 Low Spacer
This is the thin 1.0mm spacer that comes stock with all Shimano 10-speed cassettes and fits between the cassette and hub. It is required to make a Shimano 10 speed cassette functional with your drivetrain on all Shimano hubs, Shimano wheels and all non-Shimano hubs and wheels that accept Shimano cassettes. Only one is needed per cassette.
These should not be confused with the black, 2.35mm spacers that space out the smaller cassette cogs.