On a slip-on tub diverter spout, does the set screw tighten against the copper pipe or the plastic slip-on piece?

There are openings in the plastic that, if lined up properly, allow the screw to get all the way to the copper. Out of the box, though, these openings were not aligned with the screw, so it got me thinking. I lined them up, installed it, and tightened against the copper, but I ended up with a drip leak right at the screw location, so made me wonder again.