The 3/17 Occipital Left-leaning Heterodyne Wrench is a sparky tool that Agatha to try to establish a détente of sorts with Moloch von Zinzer, in the labs aboard Castle Wulfenbach. She's surprised to learn that Moloch, who's not really a Spark, knows exactly what this thing is, whereupon he explains that non-sparky soldiers can be mechanics too -- reasonably enough. This leads directly into Moloch's soliloquy on how he came to blunder into Agatha's life following his service aboard the Vienna 707. It's suspected that this bit of information is going to be important some day.
We never do see this particular wrench put to use, but it's served its purpose.