A flexible coupler with 8 mm bores on both sides.
Usually, flexible couplers have either a set screw, or a slit that forms two jaws that can be tightened with a screw. This coupler has both, which is an advantage. Using both mechanisms to tighten it helps keep it centered, especially when the object it tightens around is slightly smaller than the nominal diameter (M8 thread, for instance, is hardly ever fully 8 mm in diameter).
Overall dimensions: length 25 mm, diameter 20 mm (jaw tightening screws protrude slightly).
Material: aluminium alloy, steel.
Part number: EM-FC8-8.