One time that I saw similar damage:
Back about 1991 I put a new hydrostatic trans. in a large diesel powered commercial mower on an estate. It was the type w/ a variable swash plate pump in the top, and a swash plate motor on the bottom. They called me back a few weeks later and said it wouldn't move. The machine was in the shed, and when I disassembled the trans. it looked like that, only worse. The oem refused to warranty the unit, saying it had been towed without the engine running.
When I got to the bottom of it, it turned out they had run it out of fuel and drug it back to the shed and restarted it the next day. The lawyer that owned the estate sued me over it, but that's another subject, and part of the story of why I quit working on other people's equipment.