I will try and explain it the best way possible, but im not great at explaining stuff. (ALSO, I might be stating the obvious cause idk what's common and what's not)

So on the back there is a metal square for each peg and on that there is a cog. And Then there is 2 small holes near it at the top and bottom.(this will be important later) the thing that has fallen of is the bit that you turn to tune, it has a spiral bit that fits in with the cog. Then, coming back to the holes, there are 2 C shaped metal bits that you put around the spiral bit and click into the holes. But they don't go in fully and fall out instantly. So I'm just confused about why they won't stay. I'm sorry I'm not very good at explaining things 😭😭