This is an insert that allows you have floating shelves with no visible screw holes. It works well as long as it doesn't need to take much weight. The corner shelves can take much more weight but are a bit more fiddly to implement.

The part is screwed into a hole in the back side of the shelf. As long as you drilled the hole straight (I was doing it by hand) the teeth will grip and lock the orientation of the part. The shelf now has a "keyhole" feature in the back of it that can slot over a screw in the wall. By altering the depth of the screw in the wall you can get a nice tight fit.

Print with keyhole side up.

Directions

Using a 16mm spade bit drill holes AL 15mm deep.
Drill pilot holes for the screw.
Check CS screws will both fit in part.
Put screw in part, align orientation and tighten screw until the part is pulled flush or deeper. (put screw in first! - don't just hammer it in)
Accurately mark up the hole positions on wall.
Drill holes in wall and insert wall plugs.
Put screws in by hand and tweak the depth of each one (testing one at a time) until it pulls the shelf tight to the wall.
Plonk the shelf over screws and pull down to secure in position.

If doing a corner I found it best to have two vertically orientated screws on one side and a single screw on the other wall. Orientate this part 90deg to help get the shelf on.