The brown color seems to suggest a previous fire in this area - not a real big surprise
I have seen only one or two of those "A" meters with wiring feeding into a unilet fitting like this. Mostly, they are installed with the terminal area protruding into the top of a service switch of some type.
I'm guessing that the device to the right is a fused main disco with one of those old "rotor" contact disks. This old service really doesn't look too bad (for its age)- its all the ugly stuff that's been cobbled onto it over the years that's disturbing.
Are you doing any more work for them (I hope), other than the card reader?