I had a similar seatpost, years ago. It was a friction fit at the curved area, between the seatpost fitting on the bottom, and the bottom part of the saddle clamp. No ridges on the interface. That made it very adjustable, but prone to major slipping.

At first, it slipped when I rode over a bump, flipping the back down as far as it would go. Cranking down the bolt didn't fix it completely.

I ended up using some wet-and-dry sandpaper on both halves to roughen the curved surfaces, probably 180 or 240 grit, and a coating of Tacx Carbon Assembly Paste -- that was a tube of gel with plastic grit in the gel. (I still have the tube, 20 years later!) Assembly paste is great for aluminum stems to aluminum handlebars, too.