Truss rod recessed hex nut was stripped. Poor access to begin with. After adapting the hex nut so I could use a wide flat head screwdriver.with lots of torque, it became clear that the truss rod is frozen stuck. Also there is a fairly acute warp in the top on the bass side just above the bridge.
If you ever buy a used instrument, please please please check that all hardware works properly – especially the truss rod.
Don’t buy a bolloxed instrument!