Below is a list of sites where you can find information on the year of your Mercury outboard motor.
You will need the motor H.P. and serial number.
The serial number can be found on a plate located on top of the transom mount.
Some of the serial numbers in the lists below will cover several years.
Just interpolate to find the year of your motor.

http://www.autorepairmanuals.biz/sit...83/page/599300

http://www.maxrules.com/merc_contents.html

http://www.iboats.com/Mercury_Voltag...view_id.271572

http://www.eastcoastmarineservice.com/models.html