Let me repeat myself.
BSP's are the standard.
Your situation is non-standard.
Maybe the guy who installed it didn't have a 149 adapter, or maybe he didn't want to use a 66E terminal, or maybe he was just a tinkerer.
It does not matter, electrically, how the various components are wired. It is a matter of how you would maintain (repair) a set-up the most efficiently. If the subscriber's telephone were to become so defective that it needed to be replaced, a repairman should not have to unscrew all those wires from inside the set. He should just need to do what the BSP says to do: Unplug the set and replace it.
"My" BSP does not show what you have, because, as I have said, your situation is non-standard. No pages will show your situation.
The 149 adapter is not a 2 in 1 adapter. I suspect you mean a "2 into 1" adapter, meaning, I suppose, that two telephones can be plugged into one cable. That's not what a 149 does. If you look carefully at the diagram, you will see that the 149 is used to derive the speakerphone service leads from the telephone, at the place where the station cord and the station cable interconnect, and allow access to them.
An alternative method is to run the cords from the components into the 55B control unit, if it is mounted right near the desk. It is never OK to run them into the telset.
The numbers that you have provided actually do not help us know what mounting cord leads are being used for the set-up, because those screws are blind terminals, (they have no internal connections) and are used arbitrarily. The important information would be the colors of the pairs in the cord that are connected to those screws.
A 4-type speakerphone can be powered by KSU power, but a 3-type cannot. It requires a floating supply to eliminate ground hum. On large installations, a dedicated 20-type or 30-type supply was installed to run many speakerphones in an office. Its outputs were left ungrounded and isolated from other key service power units to avoid hum.
