What is this kind of symbol meant to be?
The dashed lines between Cd and Cz simply denote a temporary connection between the test circuit and, e.g., an oscilloscope.

And I think the circle on the right with the squiggly line in it does in fact represent a CRO (cathode ray oscilloscope), with deflection plates and the resulting displayed waveform.

Cz and Cm represent the parasitic capacitances associated with the scope that will affect the measurement.