This is ER16 High Quality Collet with 3.5mm Hole.

Specifications:

• Made from Alloy spring steel and fully hardened
• All critical collet surfaces are precision ground for accurate fit
• Works in all ER16 Collet Chucks
• ER16 collet has a full 3.5mm range of collapse while maintaining full accuracy
• ER16 collet has a range :  3.0mm to 3.5mm