This is ER20 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 ER20 Collet Chucks
• ER20 collet has a full 6mm range of collapse while maintaining full accuracy
• ER20 collet has a range :  5.5mm to 6.0mm