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Why not have material files for each material the part(s) may be made of, add them to the list of materials in the part model genericand set which material file is considered assigned to the instances by adding the material to the family table & setting it in the table? That way the mass is always calculated based on the geometry & material fileand is correct as changes are made to the model(provided you set mass_property_calculate to automatic or are sure to manually force a recalculation after changes).
Erik