Open ended sleeve designed for mounting tools with Morse taper without a tang, such as end mills.
Often used in manual milling machines.
Ground external and internal taper.
A long screw placed inside a spindle goes through the sleeve into the tool fitted into it, which is then locked with a washer and a nut.
A notch at the bottom enables the use of a wrench to block the rotation of the spindle while mounting or dismounting the tool with the spindle nut at the top.