Looks like series to me.
Yes, looking at your annotations you appear to be right (and I was wrong). It is a series diode, and is there to prevent a dark panel from draining the battery. A good controller won't let this happen anyway, but with a cheap controller or a direct battery connection the diode is probably a good idea.
I tested some of my 100W panels and they actually didn't leak significantly until the reverse voltage exceeded the panel Voc (open-circuit voltage). Other panels may be more leaky than mine were.