Impressive search skills.
I just found this:
"Some fuel injection cylinder heads also feature bracket and water inlet castings on the inlet port side which are
used for fuel injection purposes only. In order to fit Manifold(s) ,it will be necessary to cut bracket casting or to plug the water inlet, or to grind the inlet manifold casting, whatever is easiest. For preheated manifolds
caution however will have to be exerted not to break in water passage."
in the very old PF Engineering flyer on 807 engine install.
I also went through some pics I have collected from the web. One vaguely shows a pump at the
D location, another vaguely shows what looks like an electrically controlled water valve.
So it looks like, depending on version, it is either a fuel pump or a coolant course.
I'll just block it off.
Ron