In the diagram you've provided for the setup that is a control unit #3B mk II, located below the variometer. This was pretty much standard for the truck/ground station. It has a large socket for the cable to connect to the WS19 proper. It also has at least 2 drop leads for the headsets.
The grey box in your photo (the one with button marked Call Commander) is not a control unit. It is a junction distribution box #3 and it's function was stated earlier.
Other control units can be used, not just the 3B.
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