Reading Stretch's write-up (again, forgot about it) that was linked above (
http://www.peachparts.com/shopforum/diesel-discussion/303379-what-feeling-will-properly-adjusted-w123-w116-w126-power-steering-box-2.html), M-B calls the spool valve a "control valve". The FSM says to not open it and mentions no adjustments. It appears there are 2 springs to center it in the bore at a neutral position where the box won't want to self-steer either direction. I expect that those 2 springs are the adjustment and were probably hand-selected at the factory. Perhaps they were swapped during the rebuild (possible?) or springs from another box used.
Besides getting different springs, you might be able to shim one side or the other with washers, similar to what we do in the injectors. Problem is that would take much trial and error, losing a lot of fluid and even worse you must remove the p.s. gear from the inner frame each time to get at the port. Maybe only once. I would have the box in the air (Pitman arm off) w/ hoses connected and idle the engine to get hydraulic pressure. After a few passes, you could probably get the right shims. M-B probably had a hydraulic test bench and measured the off-center force so they could pick the right springs on one pass. You can find the Chrysler FSM (1960-70's) online to see how those gear-boxes adjust the spool valve (in-car, easy to get to on top), and perhaps other manufacturers are similar. martureo might even do enough refurb's to make rigging up a tuning bench practical.