The injection pump reservoir gets it's oil from the crankcase. There is no reason to check or add any oil to it. It is fed automatically from the engine oiling system.
That's the "lower" crankcase oil filler hole (versus the "upper" filler hole on the valve cover).
First time I saw that; I had to scratch my head a moment... then I believe someone mentioned that Yanmar configured the engine such that in a marine application or standbye generator or pump use, etc. - the operator could still fill the crankcase in those multiple platforms.