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 12 tooth sprockets, so they are priced lower.