This adapter is for the E3Dv6 clones coming out of China that have a M10x1.0 threaded hole where the bowden tube enters the hotend.  
I had been using a standard tubing fitting with a tapered pipe thread that kind of fit and seemed to work, but I never liked the fit.  I also didn't like the tall, heavy metal fitting making the E3Dv6 hot end taller than it already is.
There is a Chinese supplier that makes clones that have the E3Dv6 style bowden tube fitting with the clip and all (see the link below). I've got some on order, but I wanted to make the parts I already have work nicely too.
https://www.aliexpress.com/item/V6-HeatSink-Extruder-Radiator-for-E3D-v6-hotend-Remote-All-Metal-long-distance-1-75MM-for/32847326188.html
This adapter makes use of a tube fitting that is intended to be pressed into a plastic body.  You can get them at the link below.
https://www.filastruder.com/collections/e3d-spare-parts-and-accessories/products/bowden-couplings-all-types?variant=21285980676
I also threw in a matching clip since…