If four digits are present, the first two digits refer to the year (i.e., a “66” would mean 1966).
The last two digits refer to the week of the year (i.e., a “26” would mean the 26th week or, roughly, June).
Note that speakers also use the same type of date code.
For example, EIA 606-4-21 would denote the 21st week of 1964.
It is also advisable to add about 6 months to the date to allow for time these parts may have spent in inventory storage.
Several different types, in vertical and diagonal weaves, were used.
Please note that it’s quite difficult to precisely determine the production date of early Fender amps.
They are detailed reference resources with a wealth of information on determining the production years of various amplifiers and on Fender history in general.