For more insight into the monolithic stage imbalance, I measured the violin modes of the wire segments supporting the PUM. The procedure was as for the fibre violin modes, except with the laser and QPD raised higher. The beam path was a few mm above the top of the mass and a few mm below the side shelves on the LSAT. The observed frequencies were

left front 415 Hz
left back 428.5 Hz
right front 414 Hz
right back 424 Hz