There should be a serial number on the metal body somwhere (I am thinking on the exhaust port) and the first 3 digits of that number is the date it was made - the first number is the month and the other two numbers is the year.
The first number is the plant where it was produced, then the next two numbers are the year, and the next two numbers are the month. The remaining numbers indicate the unit in which it was produced that month. I hope this helps.