EIA manufacturer code 312 identifies Sylvania / ECG, based in USA. Bright green/yellow silkscreening, top D-getters, 3-mica options.
The EIA (Electronic Industries Alliance) manufacturer code system was used on vacuum tubes, transistors, and other components to identify the factory of origin. The two-letter code stamped on a tube's glass envelope or base identifies where it was made, which is a key factor in assessing its quality, rarity, and value to collectors.
Browse TubeDealer listings and filter by brand to find tubes made by Sylvania / ECG. Many collectors and audio engineers specifically seek tubes with the 312 code for their known factory quality.