Just an FYI, and someone correct me if wrong, but I'm pretty certain, they are the same, 16 gallon or 20 gallon, left side or right side, all the same.
Now there are different year model as far as style and updates due to the what was said to be, fuel tank explosion on impact issues. So do you care if they are early or late style? To the best of my recallection, the early style are like an L and straps hold the tank to the shelf of the L. Later style are more like a J, a real J not like this font but more like a hook and the extended hook portion add a little more protection to the tank. The later ones also have a stud out the bottom to be able to bolt a plastic cover over the lower side of the tank. Many end up taking that off because rocks and such gather between the plastic and the tank and end up rubbing a hole in the tank.