The date code consists of a number (from 1 to 12), followed by the letter "A", followed by another number (from 0-9).
The first number represents the month of manufacture, while
the last number represents the year. (The "A" acts as a separator.)
For example, the date code
3A4 would decode to March 1974 or March 1984 or March 1994
Note that the decade can't be precisely determined but a quick check in a Radio Shack Catalog will help you to determine a decade.