Under GRI 1 and the relevant chapter notes, parts that are identifiable as suitable for use solely or principally with a specific instrument are classified with that instrument's parts line. A component equally suitable for both semiconductor optical inspection tools (9031.41) and general optical instruments (9031.49) would typically fall under the NESOI parts line 9031909195. Documenting the OEM part number and its designated parent instrument in entry records helps support the classification if CBP inquires.

Yes — 9031904500 specifically covers bases and frames for coordinate-measuring machines of subheading 9031.40.40, separating these structural components from the broader parts NESOI lines. This distinction matters because structural castings or granite surface plates used as CMM bases are sometimes misclassified under stone or metal headings; their dedicated tariff line in 9031.90 reflects their functional identity as instrument components.