This is ER11 High Quality Collet with 6mm Hole.

Specifications:

• Made from Alloy spring steel and fully hardened.
• All critical collet surfaces are precision ground for accurate fit.
• Works in all ER11 Collet Chucks.
• ER16 collet has a full 6mm range of collapse while maintaining full accuracy.
• ER16 collet has a range : 5.5mm to 6.0mm.