Section 18 Optical, photographic, cinematographic, measuring, checking, precision, medical or surgical instruments and apparatus; clocks and watches; musical instruments; parts and accessories thereof 91 Clocks and watches and parts thereof 91 11 Watch cases and parts thereof. 91 11 20 Cases of base metal,whether or not goldor silver-plated Commodity Code Description 91 11 20 00 Cases of base metal,whether or not goldor silver-plated