This is a full set of 13 replacement knobs for Heathkit tube testers. They are quality 3D printed reproductions and are a direct replacement to the originals in appearance, fit and function. They are compatible with most models, including IT-17, IT-21, IT-3117, TC-1, TC-2, TC-3 and TT-1. If you have any questions, please feel free to contact me. Need multiple sets? Ask me about bulk pricing. Want to give your unit a custom look? If desired, I can supply them in other colors upon request!