If three digits are present, the first digit would refer the units of the year i.e. If four digits are present, the first two digits refer to the year i.e. The last two digits would refer to the week of the year i.e.("26" would mean the 26th week or approximately June).Fender never printed any since there were plenty of leftover AB763 tube charts available and these were used well into 1969.
The code follows the format: = a number from 1 - 52 indicating the week of manufacture.
These date codes will contain two letters of the alphabet which refer to the year and month of production, and may be interpreted as follows: In the absence of the rubber-stamped date codes, EIA numbers taken from the transformers may allow you to determine the date of production of your amp.
These numbers always begin with "606" , and are followed by three or four digits in various combinations.
On the inside wall of your amp cabinet, there may be a tube location sticker which shows the tube layout, model number, production location etc..
There may also be some letter date codes rubber stamped on this sticker.