Tonearm effective mass

I have read about and think I have a basic understanding of matching cartridge compliance and tonearm effective mass.  I also have read a bit about how effective mass relates to moment of inertia, and how one can alter this by different combinations of counterweight/pivot point distance.  What I havent seen, and I would think would be easier, is the effect of adding extra weight, ie heavier screws, in attaching the cartridge to the headshell...would this not, as well, increase the effective mass/moment of inertia?  .help in understanding this would be appreciated...thank you