Trick question, Ulysses?
While three conductors are typical in a balanced cable - you're right, they could have tied the shield to the chassis, I suppose (or left this end unsheilded?).
So this unit is optimized for balanced inputs but unbalanced outputs, which I guess is OK since it will likely end up feeding a channel insert anyway.
One final question then, the tie ins to the + and - 15VDC are to power those chips? (OK, two questions, should I worry that the rails are +14.99 and -14.93)?