Brace for IKEA Hejne shelves. This makes sure that they're not as wobbly, as you can only use a single bolt when assembling them. The models provide support using the inner hole, and slides over the shelf.

It is possible that you need to loosen the existing bolt a bit, then slide this model in, and re-tighten the bolts. Don't be like me, use a washer for both bolts (the ones in the photo have been replaced since).