The oilways for the the camshaft bearings are drilled from the outside, diagonally across the bores where the bearings go and continue down to the oil gallery below the camshaft. When the bearing shells are fitted, they seal off the outer section of the drilled hole (above the camshaft). The shells are drilled on one side to match the lower section of the oilway to allow oil flow from the gallery.
The rubber plugs as far as I can determine are just to plug the hole to keep water and dirt out - they don't seal oil pressure. I just use silicone sealant to do the same.
Edit: You will probably find 3 of these, below #2, #3, and #4 injectors.


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