Full project write-up available here https://www.miatafied.com/projects/posts/project-g-g-string-bikini-top/
This is an adapter for the center brace of a Project-G Miata G-string bikini top https://projectgla.com/product/g-string-bikinitop/
The threaded shaft and two nuts make the length adjustable for a perfect fit between the handle of the convertible top bow and the roll bar.
The author marked this model as their own original creation.