Perhaps a brief description of the IB3 would be helpful?
There are two sockets on each side, marked ACTIC and HARNESS. The ACTIC ones aren't used in the Sabre and should not have anything plugged into them. The HARNESS ones have cables that daisy-chain through the other Clansman boxes such as the Commander's box you've already mentioned. You seem to have a cable in the ACTIC socket on the right-hand side - move it to the HARNESS socket below it instead.
On the bottom of the IB3 there are four sockets marked A Set, B Set, C Set and 28V. A Set and B Set go to the two radios. C Set isn't used in the Sabre. 28V carries the power to the IB3. I can see from the last photo that A Set, B Set and 28V have cables as expected. Check that the cable plugged into 28V has power on it. If it does, then when plugged back in, you should hear a click in the headphones if you switch the IB3 on and off.
Andy