Hi Frank,
I think the screw in the center of the "needle" is an eccentric screw, i.e. it will not unscrew but will allow for ease of regulation when turned (like a cam). To me, it means that with a screwdriver you can gently move towards the + (in your case) that needle. Just what I think it is, let's wait for other, more experienced members and their opinions.
Cheers,
Bob