This is ER20 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 ER20 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.