Originally Posted by Brian Carlton

From the factory, the line from the banjo bolt goes to the port on the underside of the switchover valve. The line for the ALDA goes from the horizontal port at the top of the valve to the inlet at the ALDA.

The PO bypassed it because the valve was clogged with soot and he didn't want to bother to clean it. If you reconnect it, you'll also likely need to replace the sending unit for the valve. That device also becomes clogged over time and, although the plumbing looks correct, the valve won't operate when (if) the pressure reaches the setpoint..........15 psi.