I don't know if this helps or not, but Bigshrimpin posted this link in the links section:
http://www.warnersdock.com/Modelyearlookup.htm
There, concerning Mariners it says, "Mercury/Mariner does not code the model year into the model number or serial number. The model year is determined by the first serial number sold during the model year. Example: Merc 115ELPTO with serial OT123456 could be 2001 and OT123457 could be 2002."