That's a nuisance that section being missing. I did find this EEVblog thread which mentions that there are two tabs on the underside of the scope, but even this poster had some difficulty removing the board on his scope:
https://www.eevblog.com/forum/repair...-and-teardown/
Seems the arrangement is a little different on my PM3094 so it perhaps needs someone with experience of this specific model to advise.