Forgot to mention -- because of the suspicion that the transmitted input mode cleaner noise is dominated by coil driver noise, I switched H1 SUS MC1, MC2, and MC3 coil drivers into their lowest noise state (State Request 2 on M1/Top "LP ON", State 3 on M2 and M3 "Acq OFF, Lp ON). They had been in their highest-drive-strength (and therefore most noisy) state. 

Sadly, since I couldn't get the arm cavity locked on Red, I couldn't reproduce the measurement -- and we lose the arm cavity tomorrow. A chorus of sad trombones plays in unison.