Yeah Bud, that one was a big one. Heck even the plastic broken down. I think it was because of the seal around the door cover who kept the liquid and vapours inside, breaking down the plastics.
I had such happen to my 54-II but since it is not a totally hermetic one the damage was contained in the back cover and a spot on the MPU legs that was easy clean and reflown. The culpid in this case was Duracell Procell. And I has able to save the 289 before they started leaking (but they were already discolouring).