This anodized aluminum focus knob guard for the Preston HU3 handset attaches right onto the back of the hand unit, and allows you to place the handset down on a surface without bumping the knob position and changing the focus. It also protects against wear and tear on the preprinted focus rings, as they tend to chip and delaminate easily when constantly rested against. For less than the cost of one replacement ring, you can protect against damage to all your rings.
Two 4-40 screws are included (a hex cap screw and a flat head.) Simply remove the two corresponding screws from your handset and install the guard with the provided screws.