Figure 1 is the dark fringe trend data spectrogram of this week-end, it shows one pair of very narrow lines lines around 25.7 and 27.08Hz which appeared around June 11 3.50am (Figure 1) till the this lock ended at June 12, ~18am.
A closer look shows that these lines are part of a comb which is spaced at 1.354 Hz (full list of lines of this comb clearly visible in dark fringe with some 100s resolution are: 12.1917 23.0130 25.7235 27.0778 29.7840 43.3218 44.6756).
This comb looks took the place of another comb spaced at 1.644Hz
( full list of lines of this other comb are: 11.5059 18.0843 21.3728 24.6593 27.9485 31.2386 34.5192 44.3874).
Figure 2 summarizes the frequency of the two combs, and the demonstrates the combs spacing is 1.354 Hz in one case and 1.644Hz in the other case.
The "comb switch" is illustrated in some high resolution spectra and spectrograms in Figures 3 to 8.
