[hneel]

How do I open this screw to loosen the ring?


It seems just round, with no possibility to fit in a screwdriver.

This is how it looks from the other side:

[Shambles]

Looks like it needs one of these...

[Asterix]

With this tool, whatever it's called in English. Then replace with a normal clamp afterwards 

[CraigB]

Strange that it has Allen key clamps usually Allen and Star keys are mostly US parts.

[hneel]

With this tool, whatever it's called in English. Then replace with a normal clamp afterwards 

This was the way that worked. 

[cruiserfied]

Just for every ones knowledge the caps are whats left of the shear bolts when installed at factory. Pull them off with some pliers and there is a standard 8mm/flat blade screwdriver head.