A 9mm Snap-on ferrite clip for keeping RF out of keyboards/mice and other Shack perhiperals. A few loops of a cable around these can work wonders on keyboard/mouse lock-ups and also reducing breakthrough on "Hi-Fi" speakers. Does not fit RG-213.
M0PZT says: "I used one on a Logitech keyboard which occasionally locked-up when transmitting 100w of CW on 7MHz. 2 turns of the cable through this ferrite cured it..."
If you plan on using snap-on ferrites on coaxial cable (especially at HF frequencies), it's advisable to use a proper 31-mix Fair-Rite core such as the Problem Solver... Or, for smaller cables, Aerial Parts of Colchester sell these.