there are some typos from 1975 that will read as 1985.We have seen this a small handful of times and it seems that there was a week or two in 1975 where they were accidentally stamping them as 1985.Next, (if applicable) look for the date code on the tube chart.
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.
The code follows the format: XXXYY ZZ where: XXX = a two or three (possibly four on newer amps) digit number indicating the manufacturer.
(see chart below) YY = is a one or two digit code indicating the year.
If possible, open up the chassis and check a few of the pots’ and capacitors’ manufacturer codes.
Remember, many components could have been changed over the years, speakers blow, caps dry out, transformers melt down, and pots wear out.
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.
As you can see, the numbering scheme was consistent from the first Mark I in 1969, through the Mark II’s of the early ’80’s and on to the Mark V of 1984.