Mick,
Your best bet is to just fit the ANR boxes, and use modern headsets with green pressel boxes. Black pressel boxes will not work on ANR systems.
The Commanders Box Fixed is designed to be used with the Commanders Personal Unit in your photo. This box has a 7 pin plug and a 10 pin plug. The idea is you can control the box from the bit hanging round your neck, useful for changing radios etc when busy in the vehicle. They are quite heavy and although I have a CBF in all my vehicles, we normally just use a pressel box plugged in to the CB2s.
There are lots of harness cables around, you may find them at shows or try asking the dealers who sell the boxes. Usually AFVs are surplused with the intercom and harness cables in place. You may also find that the dealers want to hang on to them so they can sell complete systems.
I have some spare ones I could sell, let me know what length you need and I'll see what I have. When I did my Ferret I could only find some short cables, so I unsoldered the 12 pin plugs and soldered them back on some 12 core cable from Maplin. Not a nice job!
Chris