Mine is on a plate located under the seat.
MTD
You can get that from Sears' online parts directory.
The first 6 digits of a Craftsman lawn tractor serial number will tell you the month, day and year it was built. So, if your serial number is 1599SO6258, your tractor was built on January 5, 1999.
MTD
March 01 2004
1997
Husqvarna
I found this page, maybe it can help? http://sears.pammar.net/id_number.html
Contact your local Sears outlet.
You can get it here:
The John Deere lawn tractor with the serial number A660F 070881M was manufactured in 1985. The serial number indicates that it belongs to the model year 1985, as John Deere typically uses a specific format for their serial numbers to indicate the production year.
Go to the sears web site and enter you model number they have PDf's for these.