These are protective windows for your Optoskand multi kilowatt fiber cables. Shown of top of an end piece to the fiber for illustrative purposes. You will receive the cap part only. The metal part of the cap is 11.95mm in diameter. The whole assembly is 14.18mm tall. it is threaded inside but I don't know  how many TPI it is. The inner dimension on the threads is 10.2mm.  The quartz window is AR coated. (I'm assuming it's quartz but it could be some exotic substrate that can handle 10KW of power).  I have 3 available. I opened one for photos the opened one will be sold last.