The Bon Musica Shoulder Rest Knurled Nut keeps the hooked section of the Rest attached to the whole Shoulder Rest.
The Hooked Section is the most distinctive part of the Shoulder Rest. It is the wide part of the Rest, which hooks over the player's Shoulder. As this is a vital part of the Shoulder Rest, it is designed for maximum comfort.
The Knurled Nut is of the utmost importance to the Shoulder Rest, and it is Much More Convenient, (And Much Cheaper!) to have the ability to buy spare parts, rather than having to replace the whole Shoulder Rest in the unlikely event that your original Bon Musica Shoulder Rest Knurled Nut should come loose and be lost.