It does look like it is for the removable SS rod holder. I've been wanting to put a rod holder up around there for holding the net when I'm fishing, but don't really want a one sticking out all the time. That would be kind of a perfect mount. It's there when you need it and stored away the rest of the time.
I do think its for the rod holder, but the question is what did the previous owner use it for? It is in a rather awkward spot for salmon fishing. Halibut jigging maybe, but rod holders can hold many other things. Can't you even get those kettle type bbq's that stand off from the boat and fit in rod holders? That would be cool right there.
Jackie