Dating celestion guitar speakers
The T1221 was the stock speaker in Marshall lead guitar 4×12 cabinets (1960 model cabs) from the mid 1960’s up until the late 1970’s and was also used by many other brands at the time.
The year is confusing because it is only a single digit.
In later years, the EIA converted to a 4 digit date code so the year could be easily identified.
There are 2 acceptable methods of displaying the EIA date code, with or without a hyphen between the manufacture code and the year/week code.
Anything else is at the discretion of the manufacturer.
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The T1217 quickly superceded the T1134, so they may have only had a production run of about 12 months or so. They usually have pulsonic cones stamped H1777 or 003 with the welvic doping on both sides of the cone.