Riccardo,
I THINK, I understand your problem.
The brass blade adjustment knob is corroded onto the threaded “screw” that attaches to the frog and when you try to turn the adjustment knob the threaded “screw” unthreads from the frog instead of moving along the screw portion.
You need to get two nuts that are the same thread size/count that will go onto the end of the screw that threads into the frog. They may not be a standard thread type, you’ll just have to try some sizes.
Thread the two nuts onto the end that goes into the frog and tighten one against the other. This gives you something to hold onto while trying to unscrew the brass adjustment knob. The nuts can be held in a vise or with locking pliers.
Hopefully this will give you enough leverage to break the knob free. It may just be really dirty.
If it doesn’t yield, you’ll need to apply some heat to the knob and some penetrating oil to the threads.
This turned out a bit more long winded than I wanted, hope this helps.
Craig