A Super Echo Twin with serial number 404553 was made in 1964 (4), during April (04) and may have been the 553th amp made that month (553).Caution is needed for dating amps from 1953, 1954, 1955, and 1960.Ampegs can be divided into six distinct groups for dating purposes: pre-1953, 1953 to mid-1965, early 1965 to 1969, 1969 to 1979, 1981 to 1984, and post-1984.Each group uses a unique serialization scheme that can be used to assist in dating the amps, but in many cases, it is the features and characteristics of the amps that determine the year of manufacture.The Nth unit, however, may not have started with 001.Ampeg may very well have used 100 or 101 as the starting point during slow production periods.Of course, these codes are only applicable to original components, not replacement parts.
I pulled this information from Google's cache of the site.
These codes have been used on components including potentiometers (pots), transformers, capacitors, tubes, and speakers.
The code usually consists of 6 or 7 digits such as 137634 where the first two or three digits is the EIA code for the manufacturer (137 denotes CTS), the fourth digit and sometimes fifth digit denotes the year (in this case it could be 1956, 66, or 76), and the last two digits denote the week of the year.
Electronic Industries Association (EIA) codes can also be very useful for giving clues as to an amp's age.
These codes can be found on speakers, transformers, pots, capacitors, and multi-section electrolytic "can" caps.