The silver arms are just a tapered fit into the rest of the mounting bracket, there's flats on it for a spanner. They can be difficult to move though, I've used plusgas, spanners, a bench vice, and a pipe wrench before to free the bas***ds off!
Once free though, you can set the mounting hardware in the right position, then tightening the top bolt will pull the silver arm in again and keep it in place.
Looks like it could be a bit awkward to set up with the shape of the fork itself though, but not impossible.