convention for drain/source position?
Considering that this mosfet is connected as an analog switch, it is possible for either side to take the role of the source and the drain. The convention for an NMOS is simply:


The source is the side that has the lowest voltage.
The drain is the side that has the highest voltage.


For a PMOS it is the other way around:


The source is the side that has the highest voltage.
The drain is the side that has the lowest voltage.


It is perfectly possible that the drain and source change sides constantly throughout circuit operation.