You are looking at a used Collet Chuck Attachment for a Lathe with a D1-6 Mount. It appears to take a 16-C collet, however I'm not certain.
I know its larger than a 5C. The dimension of the opening at the end at the outside of the taper is approximately 1-13/16ths inches. I do not have a machine to put this in to test it so I have never tested the item to see if it opens and closes properly.However, the threads down inside the unit look to be o. As well as the key inside the unit.
You can see the grinding on the inside is still in good condition. There is a square headed wrench hole down on the front of the unit and it is geared to the threads down inside the unit. I have never taken this unit apart and I have never tested the accuracy of this unit.
The D1-6 appears to be o. But I have never tested it. Be to ask questions before purchasing.The item "Collet Chuck Attachment for Lathe with D1-6 Mount Used" is in sale since Thursday, October 10, 2019. This item is in the category "Business & Industrial\CNC, Metalworking & Manufacturing\Metalworking Equipment\Metalworking Lathes". The seller is "rnl03" and is located in Shelby, Ohio. This item can be shipped to United States.