My pictures are embarrassing next to your well lit photography, but at least these ones load. I think I can upload the two pics of the complete motor and transaxle in another post.
The rear transverse passage is a coolant flow balance port. Aurelia motors had one too. It was supposed to help ensure even and sufficient flow. Silting in the V6 motors would tend to cause rear cylinders to suffer overheating before forward cylinders. Regular draining of a sufficient amount from the lower coolant tap was advised so as to flush silt.
The front with nose.
Top view, vertical coolant hole is upper left by edge of cylinder bore #5.
Rear of block (notice empty crank bore), transverse coolant port visible. LH plug seals hole to #5 inner water jacket, same on RH to #6.
RH water jacket with hole from transverse port to cylinder #6 (not quite visible in upper picture).
LH vertical passage by cylinder #5 down to transverse port.