Here's a really nice Jacobs Rubberflex collet chuck. It has an L-1 mount that fits lots of American made heavy duty lathes. It's model is 91T-1. The OD of the threads is 6.
This one wasn't used very much and well taken care of. Most of these get pretty beat up on the aluminum.This one has a few minor nicks. The body is smooth and clean and only has some very light discoloration from humidity on the steel parts that would polish off easily if someone wants to.
The mounting taper is nice and smooth and the threads are clean and crisp. Overall a very nice item.