Comment by ZiiS

Lots of liquid damped compasses do not have a transparent base. The liquid is very good at protecting the needle (induction compasses often use a lock), prevents condensation, stabilises temperature, and is noncompressible for diving. Induction compasses tend to be used for fast reading whilst off-level, so tend only be useful for sighting compasses. TBH I am not sure even map compasses grain a lot from transparent dials, it is more that they are making the baseplate and top from transparent plastic and have no need to make the bottom from something else.