2 generator cranking batteries?

This is a follow-up as I think I've got it figured out. Basically the factory diagrams were wrong and Frank was right. The "extra" battery on the port side is connected to the single, group 27 on the starboard side which together actually cranks the port engine. So in this configuration, there are two banks of group 27 batteries that crank each engine. And one doubles as the house bank. In my case it was additionally confusing because the mystery battery was installed by the previous owner and is a deep cycle battery. Completely the wrong type. But now that I know what it does, I'll get the proper one in place.