Hi Jeff,
One possible way to ID you collet chuck is if it has a back plate like the old ones for the SL/DB Unimats then it is probably a E-16/ES-16 holder. If it is one piece and screws directly onto the U3 spindle it is probably for the ER-16/ESX-16 collets. Another visual is to look in to the nut and see if there is a eccentric protrusion near the opening. If it is there the holder is for ER/ESX-16 collets.
Cary
