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: 

YEAR                  MONTH
A ..........1951      A .......... January 
B ..........1952      B ........... February
C ..........1953      C .......... March
D ..........1954      D ..........April
E ..........1955      E .......... May
F ..........1956      F .......... June
G ..........1957      G........... July 
H ..........1958      H ..........August
I ..........1959      I ............September
J ..........1960      J ............October
K ..........1961      K .......... November
L ..........1962      L .......... December
M ..........1963
N ..........1964
O ..........1965
P ..........1966
Q ..........1967
R ..........1968
S ..........1969

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. If three digits are present, the first digit 
would refer the units of the year i.e. ("7" would mean 1967). If four digits 
are present, the first two digits refer to the year i.e. ("66" would mean 
1966). The last two digits would refer to the week of the year i.e. ("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

220xxx = Jensen
465xxx = Oxford

The next number(s) is (are) the year.  Initially this was a single digit, 
latter that was extended to two digits for clarity.

The last two digits are the week of the year.  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.  

Note that speakers also use the same type of date code.

Always remember that pots, speakers and transformers may have been changed!!

For more info, check out these references:

"Fender Amps, the First Fifty Years" by John Teagle and John Sprung
"How old is that Fender in the Window" by Richard Smith
Fender's web site -  www.fender.com