Ok, so I did another experiment, and the result was a bit surprising.
I connected the output of a GPSDO to SA124B and BB60D.
The initial drift measured by “Frequency Difference Meter” was 0.1Hz and 34Hz respectively.
I let them run for 2 hours. I expected some drift on BB60D, but to my surprise, they both still had the same drift as before!
Maybe I should measure over a much longer period like 24 hours or so.