The unit does not label which wires go where. No instructions. Which screw is positive and which is negative?

For best safety use the center connection for the hot wire.this will keep the neutral wire on the ring(inside aluminum ) side of the holder. you are less apt to get a shock if changing a bulb with the power left on.

Sockets are not polar, the wiring does not have to be specific, the positive to one screw and the negative to the other screw.