The gearbox identification is located on the starboard side of the tranny on the side of the blackened section of the unit. you should find a number like 016301211C. This number will vary depending on the model of the gearbox.
On the top forward area you should find a designator like AAZ or APU. There many of these codes that tell you what the gears and final ratio of the box are. You can search the forum threads for a listing of the code you find on your box.