The comment was for dual channel vs single channel as shown in your images.
You populate A2 and B2 when using two sticks of memory. Most likely if your were to purchase another matching set of sticks you would be able to drop them in A1 and A2 without issue. Your co-pilot which is the manual would have shown you the way. There's always a reference to what slots to use in certain scenarios listed in the manual.
From your MB's quick install guide.
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As I have stated I did try A2 and B2, MB didn't pwr up at all. Fan would spin up at full speed and then slow down. I also didn't heard any beeps.
I also try just A1 by it self and B1 by it self. Yes, MB didn't pwr up and no beeps.
However B1 and B2 has worked by it self and another memory in that other socket with same memory channel.
Now I can only populate memory channel A with single memory module or both socket Like A1 & A2, and it works.
The current configuration is A2 & B2.
I have also tried A1 and B1, that is also works.
Lets look at CPU and socket.
When I took the CPU out, to see what is the culprit is i.e. bend pin on socket, which I still don't think it was, please look the imprints on CPU pin points.
I am re-orienting the picture for easier comparisons again.
Now there was something which was causing memory channel A not to work then, it is working now.
I cannot figure out what was that, I am not worried, but I am concern.
I also understand, there is less of a reward trying to figure why Fly did sit on that wall