Meredith,
     I am glad that you discovered the absence of vertical sensitivity
in your system.  Otherwise I would have to revisit some foundational
physics!
About the offset pivot--perhaps you have discovered something
significant that nobody seems to have noticed.  Internal friction in the
rod holding the lower mass is bound to be of greater importance than
most people would expect.  Perhaps because of the bending stresses
associated with the offset pivot, there could be some 'locking-up' of
dislocations in the rod and thus a reduction in the damping that derives
from this contributor.   One would want to be sure that your improvement
is not simply due to the better properties of the ruby balls, compared
to what you were previously using?.  Ruby differs from pure aluminum
oxide crystals only in the nature of the color centers due to impurities
(chromium for ruby, titanium for sapphire; although usually the pure
material without substitutional impurities is also called simply
sapphire).
    Can you readily modify the system to measure the damping as a
function of the amount of offset?   It would be an interesting study
with potentially significant consequences to seismology.
    Randall