
The EIA code is usually followed by a four-digit date code in a year-year-week-week format. For example, 322 6104-1 means Tung-Sol (322), made in 1961 (61), week 4 (04). The trailing digit after the dash is a shift or line code and doesn’t affect the date. On a tube marked 274 5823, read it as RCA, 1958, week 23. From around 1960 on, some makers switched to coded letter dates (for example KE 188-5 on GE tubes). These are harder to decode, but the EIA number next to them still tells you the maker.
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