Remember the shadow cast on the lake which resembled a fish? You saw it through the viewing device located in the blue viewing chamber with the throne. This symbol wasn't associated with a Rivenese number, but the other numbers from 2 to 5 are already spoken for. So, it must be the first!
You saw this animal shape when you filled the stone basin. The wooden eye in the basin had the Rivenese number 2 on the flip side.
The very first wooden eye you came across had the Rivenese number 3 on the reverse side. When you walked into the cave and turned around, the mouth of the cave formed this shape. Ribbit!
Two of these were sunning themselves on a large rock. You found the associated wooden eye amongst the luminescent mushrooms. It had the Rivenese number 4 on the flip side. There was no animal shape anywhere in the vicinity, but one of them uttered a distictive bellow (sounded like ... Gehn!), which is the same sound made by this wooden eye when it is rotated.
This is the shape of the rock the beach beasts were sunning on. The wooden eye here had the Rivenese number 5 on the reverse side. This killer fish is used to execute Gehn's unfortunate prisoners, using that strange device with the harness you found in the middle of the lake!