Individual First (Top) Position sprockets for Shimano cassettes. 8 or 9 speed compatible.
These sprockets have a built-in spacer.
8-speed top-position cogs work OK with 9-speed cassettes, even though the spacing is different, because the indexing for the smallest sprocket is regulated by the high-gear limit stop of the derailer, not by indexing of the shifter as with the other gears.
We have an oversupply of 11 and 12 tooth sprockets, so they are priced lower.