Note that speakers also use the same type of date code.
Always remember that pots, speakers and transformers may have been changed!!
So, you can't afford that '54 Strat, or that '64, or even that '74 for that matter.
("26" would mean the 26th week or approximately June).
Here is an example: EIA 606-4-21 would mean 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 storage.
There are other methods used to date "Silverface" amps, you will need to get the date codes from your speaker frame, tube chart or pots.
Pots and speakers are usually stamped with a 6 or 7 digit code that can be dated as follows: The first 3 are the manufacturers code 137xxx = CTS 304xxx = Stackpole 140xxx = Clarostat 134xxx = Centralab 381xxx = Bourns 106xxx = Allen-Bradley Speaker Manufacturers codes 67xxx = Eminence 117xxx = Credence 137xxx = CTS 220xxx = Jensen 270xxx = Quam 285xxx = Rola 328xxx = Utah 391xxx = Altec/Lansing 433xxx = Cleveland 465xxx = Oxford 649xxx = EV 1098xxx = Pyle The next number(s) is (are) the year.
Initially this was a single digit, latter that was extended to two digits for clarity. Note that if these digits are larger that 52, it probably is NOT a date code!
Thus a pot with the code of 137634 would be a pot made by CTS in 34th week of 1966.
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.
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.