Isolated DC-DC common mode choke without shunt?
There's always a connection to GND, even if it's an AC only connection. For example, that C3 of yours is explicitly drawn in. But it's always there, even if there was no discrete capacitor there, by virtue of the fact the transformer inside has two charged surfaces with a gap in between (i.e. the primary and secondary coil).
Same thing with input to GND and output to GND.
Also note that if you connected one of the coils on the common mode choke in reverse it turns into a differential mode choke. For common mode choke, you hook up the two windings such that they oppose each other when current flows from left to right in both of them. FOr differential mode choke, you hook up the two windings so that they oppose each other when current flows left to right in one, and right to left in the other (which is basically just adding an inductor onto one of the lines, except it's spread out amongst two different locations).