I really should check things better before I do stuff...for some reason I welded up just one of all the odd holes on the underside, and of course it was needed for something. The something in this case being a handbrake cable bracket. Well, well. The hole now matches one on the other battery box, which isn't used for anything...
The effect of the schutz failure shows up good in this picture, with an extremely uneven final paint surface.