In the morning we were wondering what the SRM composite mass looks like, in case we want to drive it to find out the frequency and Q of the first internal mode.
https://dcc.ligo.org/LIGO-D1200886
There is a ring shaped holder which has two nubs and a set screw. This holds the 2" optic. This holder is then bolted into the larger aluminum mass. Two views from the solidworks drawings are shows with some parts made transparent. If this drawing is right, I expect that the resonant frequency is in the 500-1000 Hz range.
Here is a picture of the actual SRM surrogte optic in it's inner holder (the LLO one shown). The 2" x 0.75" thick optic is held in the ring by what I believe are 2 black carbon peek set screws which are not shown in the assembly drawings.