How it works: (see photo of back of easel and drawn top view)
1. Bracket (1.) is permanently mounted parallel to the center post and guides the shelf keeping it relatively horizontal and as it moves up and down.
2. Bracket (2.), also permanently mounted, keeps piece (3.) parallel to the center post.
3. Piece (3.)ís sides are tapered so that tightening the screw with the black handle pulls the piece towards the handle compressing the center post against bracket (1.) causing all three metal pieces to effectively act as a friction brake. When the screw is loosened, piece (3.) is pushed away from the black handle so that the narrower portion exerts less force against the center post, allowing the shelf to move up and down again.
These donít appear to be a standard type of bracket available in retail stores, but the pieces could be removed from the easel and taken to a hardware or big box home improvement store to see if the clerks could identify them and, if they donít stock them, special order them or provide information on where to buy them.
Another possibility is to contact the manufacturer and ask if the set can be bought from them.