The difference in the number of LEDs on the two boards is used for tail light / brake light change in brightness. For 6 volt, six LEDs wired in series will create a bright light but seven LEDs will create a dimmer light for all seven. So under normal running conditions with only the tail light on, the wiring controls seven LEDs. When the brakes are engaged, the power wire bypasses the 7th LED so that only six LEDs are activated. Those six LEDs are then brighter and function as the brake light. The fact that there are three extra resistors means there are three circuits of seven LEDs on the board for the brake light.
I think......
