I measured the AC and DC channel transfer functions of the eLIGO L1 TCSY ISS board for PD1 and PD2. The gain is quite high on the AC channels so I added +40dB of attenuation to the source from the SR785. As Frank pointed out, even though this isn't exactly +40dB at low frequencies, it still attenuates and that attenuation is common to both the input to the Channel 1 of the SR785 and the input to the ISS board.
The results are shown in the attached plot. I didn't bother including the phase, I'm just interested in the magnitude for calibration purposes.
The original data files from the SR785 are attached below: