The fitting is for a timing device that no one seems to have. Most timing devices you use now just go around the plain part of the tubing.So to get back on topic. I apologize for the incorrect cylinder. Yes it is #4. What does it exactly do and why only one cylinder? How does it affect injector timing?
Just like you put a timing light on #1 sparkplug wire, to set the timing on a diesel they put a sensor around the fuel injection tubing to sense the fuel injection pulse. It actually senses the line shake as the fuel is pushed through at 1800 psi.