serpentarius

Hello,I have HKS Hipermax II suspension for my 98 silvia, and I am wondering how I set the stiffness? I have figured out the front. It is the purple knob at the bottom.
But now I am confused about the rear. There is no knob at the bottom. Rather, on top, there is what looks like a hole for a hex key:

This picture is not exactly like mine - mine is all silver. Anyway, I have a hex key:

But this key is simply too small for the hole. The hole also seems to be more circle than hex. Now, am I on the right track? If this is the knob how do I turn it?