I have also never heard and explanation about why the upper button should be at the CG.
Just off the top of the head at this late point, I would think putting the upper button at CG would put minimum stress on the upper button and maximum on the lower. If the upper button was above the CG, then I would expect the stress on the upper to be a little more, but still less then the lower button and that would be reduced.
Just off the top of the head at this late point, I would think putting the upper button at CG would put minimum stress on the upper button and maximum on the lower. If the upper button was above the CG, then I would expect the stress on the upper to be a little more, but still less then the lower button and that would be reduced.