These are codes often seen stamped on components found in US made vintage tube equipment.
Code formats were not completely standardized, but a little deciphering will generally yield the info of interest. Note that many makers also stamped OEM (Original Equipment Manufacturer) part numbers above or below the EIA code, in some cases parts makers or the customer elected not to include the EIA codes, so not all parts have them.
They generally follow the following format: (example)
where 137 is the manufacturer code (in this case CTS), 59 is the year (1959), and 04 is the week.
One alternate scheme is:
274 is the EIA code (RCA) 9 is the last digit of the year (in this case 1949). Usually single digit date codes are from the 1940's, but have on occasion shown up in later decades, even up to the 1970's. If one has a general idea of the age of the piece in question, usually the decade can be divined from that.
Another alternate is in the format following:
188-5 69 32
188 is General Electric. 5 is the plant code (in this case, Owensboro, Kentucky). 69 32 is 1969, 32nd week.
And another is like this:
137 is CTS, 6104 is 1961, 4th week, 1 is the shift code (1 would probably be the day shift)