The take home lesson is that there are special order Ars amps out there that vary somewhat in their configuration, and if you are buying used, you should verify based on s/n that the one you want is right for you.
Swampwalker (System | Threads | Answers)
A similar thing happened to me on a used component. I discovered it when something wasn't operating as it should. I asked the manufacturer if he had done the modification. "I can't recall", he said. I asked the original owner if he had requested the mod? "I dunno", he said.
Nonsense. How could the owner or the manufacturer not be aware of a non-standard configuration?
Ultimately, I corrected the problem with the manufacturer's guidance.
As Swampwalker said, buyer beware, and ask very specific questions when making a deal. Assume nothing.