Another odd comment... I haven't yet tried to see what happens when my headsets are connected to a power source to activate the intercom-powered ANR as I don't yet have a functional intercom system assembled, but I did notice this:
I bought two Racal 5000/1/6400 headsets from different sources. Both are new. The older one (by serial number) is only MOD 1. The newer one has MOD 1 & 2 checked. With battery powered talk-through enabled, the MOD 1-only headset faintly passes sound through. Basically, it's enough that if someone is directly talking to you, you'll understand them clearly. As well, you can identify where mid-volume sounds are coming from. Anything at a whisper or a light tap will be muffled out. With the MOD 2 headset, everything is loudly amplified on battery only. I can hear every faint sound in the room, voices come in crystal clear from anywhere around me. I have very full situational awareness while wearing the headset.
The MOD 1 headset did also have a connection problem...the left ear kept cutting in and out, so I had to disassembled the headset and reassembled it to improve the connection of the internal contacts (it uses spring-loaded pins). So, I can't be 100% sure that the lower audio volume on the 1st headset isn't in-part due to further electrical issues. It does seems like it was reliably working after I reassembled it, so I'm more inclined to believe that the difference in performance is due to the different MODs the headset was manufactured with. If anybody wants to comment about what MODs are punched on their headset and how they perform, they may help to confirm or deny this hypothesis...