What is the purpose of these P+ and P- connections?
You missed the common point on the right board.



Figure 1. Second board has two strings of LEDs. I've marked up the 11-LED string in orange.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 2. Layout of first two boards.

It's a weird layout alright. Each board has eleven LEDs in series between + and - but spread between the two boards are another ten LEDs also wired in series between + and -.

Whoever designed the board mustn't be a regular reader here!