This holder is meant to hold a ~45mm rubber drain stopper. It attaches to the side of the drain using the M6 bolt the chain is attached to. (Hole diameter is 6.5mm or about ¼ inch).
There is a slot in the bottom that allows the chain to fall properly, as well as provide some flex to the structure to go along with minimal bending of the sink outer wall.
The author marked this model as their own original creation.