The last two ring downs I measured today showed a weird behavior of the lowest modes:
Although I'm not 100% sure, I suspect this is related to the fact that the beam reflected from the black glass was so close to the beam reflected by the disk that I could see interference.
So I broke vacuum and improved the setup, adding a peek washer below one edge of the black glass, to wedge it. In this way the reflection from the black glass is largely separated: it misses the upper periscope mirror and it is dumped on a black panel (together with the viewport reflection).
I realigned everything, installed back the disk and started pumping down at 1:30pm.