You said that you were working on a 510 but the diagram is that of a 500. I believe that they are similar enough for this discussion though. The instruction from SS for the Mark 7 upgrade requires that the pressed in legs be removed to install the new legs. There should be a pin that can be driven out and replaced in order to install the new parts. Check to see if the pin is loose. If it is then all that mey be required is to tap that pin to seat it properly. There may be someone here that has done an upgrade that can give you the complete procedure from the upgrade manual so the pin and the leg can be tightened.
Bill V