Not much in them at all a load bank that is adjustable and a few components and switches, really simple. I will take a pic of the mil one in a bit it is open.
The carbon piles in those are HD for sure. The old VAT-40 from the dealer I worked at 30 years ago ended up in a friends transmission shop. He uses it every day (as needed if course) It must be 40 or so years old.