If the article incorporates any light source — LED, fluorescent, or otherwise — as an integral component, it falls under the illuminated sign provisions of heading 9405 rather than 8310. A plain metal plate, even if it is designed to be backlit by a separately supplied fixture, remains in 8310 as long as the light source is not part of the article itself.

No — heading 8310 is limited to base metal articles. Plastic letters, symbols, or sign plates classify under Chapter 39 (plastics), and wood versions under Chapter 44. Only articles whose essential material is a base metal (steel, aluminum, brass, etc.) belong in 8310.