Ok, what's with the reverse polarity deal here? What is the benefit? I'm guessing I would have to wire the 5100r different from the 5100 when running it as a second battery? So the black is actually the +, and the red is actually - ? Is the point in this so you can wire it in series?

positive is still positive and negative is still negative, the terminals are just reversed to fit specific applications. For instance a 92-00 civic uses a 51R and a 01-05 civic uses a 51, same battery just with the terminals on opposite sides.

Ahhhh ok, makes sense