It's usually a 6 or 7 digit number depending on the year of manufacture. The first 3 digits is an EIA code for the manufacturer. The remaining numbers will be a year and week number.

For example 606-402. 606 is the EIA code for the Schumacher company. The 4 is the last digit of the year and the 02 means the second week. Knowing a little history of the model helps with the year - as in this case the amp wasn't made in 54 or 74 so it has to be 64.

If you have 4 numbers (for the date code), then the first two will be the year taking some of the guesswork out. Such as 7032 would be the 32nd week of 1970.

Make sense?