The "lens" you held at 25:35 may not be the output mirror. Hear me out.
What you described in the video is a basic Fabry-Perot resonance cavity. These things are insanely sensitive to the distance between the mirrors, which leads me to believe that the 1% transmissive mirror is fixed within the black casing.
Now, I have three hypotheses as to what the thing you were holding is:
1- Wavelength filter (basically a band-pass filter for optical signals).
2- Waveplate (a piece of glass that have different refractive indices along its x-axis and y-axis). These, with a combination of other optical components, are sometimes used as optical isolators that prevent unwanted back reflections.
3- The actual acousto-optic modulator, which means that the two perpendicular barrels are piezo-electric transducers.
Of course I could be wrong in all fronts, but I think if hypothesis 3 is correct, you could do a few fun optical experiments.
Love the channel, been a long term viewer.