Just wondering this because most I've seen you're supposed to lift the socket and point the arrow north. But how does it do anything, because the photocell can fit on 3 directions as far as I can tell because it has three prongs all the same. Since sun rises in east and sets in west, I don't know how it would have the sensor properly focused.

if not pointed north your light can come on way early at night or turn off way late in the day. 

there two type of PC sockets one is tool less socket
those type just lift PC socket up and rotate to direction
closest to north.  the oher type is loosen the screw on
top of PC socket and rotate socket around for PC to point north.  Then tighten the screw up to lock it in it place.

yes it has 3 prongs but one of them is wider and only fit
one way into the PC socket.

That is because, twist lock P-Cells have a "north" on top of P-Cell. So it would be put in the right way in the P-Cell socket. So it really doesn't matter if it faces north or not. So if you put it in the wrong way, a disastrous result may happen or may not work. Remember, a P-Cell contains three wires, black for hot, white for neutral, and red for load.

The sensor window should never be exposed to a direct sunlight, because it then degrades over time (this is the main aging mechanism of the classical CdS based units and why they are gradually switching at higher and higher lighting levels).
For the sensitivity it does not matter, as the switching happens when the sun is already/still (evenings/mornings) behind the horizon, so it has to suffice with the diffused light anyway.
Because for the diffused light the direction does not matter, directing it North (on the North hemisphere, of course) means you get minimum sunlight wear.