Back to the woodworker shop.
This is a homemade router bit extension :
Sorry, the photos are not so good.
It was for making a neck pocket in an electric string instrument I was doing.
I needed to reach a deep bottom while using a template and a copying ring.
My router bit was too short, so I quickly did this extension on the lathe, and it actually worked well.
I was fearing it would have not turned true enough, but no, I had no problem and did the job.