This is the part where I just feel dumb now don't I? Thanks!! I tossed and turned trying to suspect various things. Oh well ...
The signals are multiplexed, meaning that not all LED's are lit at once, but sub-groups are lit and individually switched quick enough that you can not tell. The SE8C uses the two commons to do the multiplexing. When one common is powered, the LED's connected to it can be lit individually while all the others remain off. The SE8C will then turn off that common and turn on the other, and control the LED's connected to it. The SE8C does this continually, switching between the two commons so fast that the LED's that are turned on in each group appear to be on continually.
So I have been thinking hard about how the SE8C uses 8 out of the 10 pins to drive 4 heads, ie a total of 12 LEDs. Seems like the "Common Cathod" is different for each of the two pairs. I guess I get that when wiring a single plant because there is so much correlation between aspects of the four heads. Ie if A1 is Green then C is always red, etc.
But can one, 10-pin connector really independently drive 4 aspects on 4 heads using the described 4 addresses?
I am still not "getting it" in my head.
NP.