As a first step towards a fast shutter at the AS port, I swapped the QPD transimpedance amplifier box for ASC-AS_C for one with the SUM output spigots for the shutter input.  From rack R5 slot 20:

Out: S1102836

In: S1301506

I also changed the P3 jumper on the OMC PZT LV driver board to the correct position (across pins 2 & 3).  Now, a 0V input to the 'shutter logic' input on the front of the OMC PZT driver box is recognized as a 'fault' condition (i.e., with 0V input, the PZT shutter function is triggered).  This draws the PZT1 drive down to 0V (from 10V).  It also pretty well disables the dither output to PZT1, so no OMC locking until we connect the PZT shutter input to a 5V supply (like, for example, the shutter logic controller on top of ISCT6).

It looks like the ASC-AS_C sum is about the same as before the swap, but we probably need to redo the dark offsets.