Axminster Non-Skid Holder.
Made to hold waterstones up to 210mm(8.1/4″) in length. Rubber holders at either end, one fixed and one sliding, which can be tightened by means of two knurled screws to hold the waterstone firmly but gently. A central rubber block provides extra support. Grips on the underside prevent your waterstone walking away whilst you are trying to sharpen a blade. Also suitable for oil stones and diamond stones.