Well, I gave it another try, and it turns out I was wrong. The 4 mm hex key fits just fine, and the truss rod turns without any problems. It just did so without any resistance at all. Now I have to admit this is the first time I’ve ever tried to adjust a truss rod, and I was expecting it to take a considerable amount of force. As it turns out, it was already all the way loose. When I tried turning it clockwise, it started getting tighter and it took a little more force to turn it. So it appears to work as it should.
So now the question is: with the truss rod all the way loose, the neck is pretty much dead straight, as far as I can tell. I lowered the action quite a bit last week, and I’m getting a little bit of fret buzz when playing the higher frets (12 and up). So I wanted to loosen the truss rod to get a little more relief, but that turns out to be impossible. Guess I’ll need to raise the action a bit more… But first I’ll replace the strings with the stock D’Addario EXL170’s.