The interphone system shown in the ARC-5 HF manual installations is the AN/AIC-4. There's an AM-40/AIC-4 interphone amplifier on the bay, item 1060232395 - these are fairly uncommon in my experience. I have the manual on it for anyone who needs a copy. I have no connection to the seller - just noticed it and thought someone might be interested. 73, Mike Hanz KC4TOS