Add my thanks to Marvin for the photo. I saw the summer IC, pulled up the schematics and there it was. No wonder the (not totally read) schematic wasn't making sense. I'm not familiar with DAC circuits, so I also must ask, is it normal for the U15/U17 opamps to be biased with such an offset?
So I guess they wanted a higher balanced signal to convert to single ended with RT6863?