Since it is a Silverface, it must be post 1967. To check the date, there are a couple ways to figure out when a Fender amp was built.

Look for the manufacturing date code, found on the tube chart to figure out the production date. In case you don't have a date code, turn the amp belly up and check the speakers, transformers for their date codes.

Also if you are feeling handy, take the chassis out (it slides out from the cabinet) and check a few of the capacitors (the tubular components on the board) and the potentiometers (knobs) date codes.

Be careful when playing the amp. The years in the attic might have taken a toll on some components.

The manufacturing code (stamped on the tube chart) is made of two letters. First indicate the month, the second, the year:

Date Codes:

Letter              Year               Month
A                     -                     January
B                     -                     February
C                     1953                March
D                     1954                April
E                     1955                May
F                     1956                June
G                     1957                July
H                     1958                August
I                     1959                September
J                     1960                October
K                     1961               November
L                     1962                December
M                     1963   
N                     1964   
O                     1965   
P                     1966  
Q                     1967

So, for instance: "JG" means the amp was made in July 1960.

Hope this helps!