Not sure what I'm seeing here, but could it be a relay for the starter? These Yanmar's commonly have problems starting if the wiring or connections are not in perfect condition and a owners commonly fit an additional relay that solves the problem.
Perhaps it was put in there to preempt the problem from appearing? The usual symptom is clicking but starter not engaging. I have all the necessary bits on my boat, as I wanted to do exactly the same, but since my engine starts fine as it is I never got round to installing them.