I've seen these before but I don't know what they're called. It's a two piece insert that creates a through-hole between them.
I drew a picture to help identity it!
Engineering Stack Exchange is a question and answer site for professionals and students of engineering. It only takes a minute to sign up.Sign up to join this community
If it is a threaded two-piece item, you'll find it by searching "binder bolt" among other terms:
Image above courtesy of The Home Depot, multiple sources abound, various length are manufactured, various diameters too.