dbus-event-serial-number is a compiled Lisp function in `dbus.el'.

(dbus-event-serial-number EVENT)

Return the serial number of the corresponding D-Bus message.
The result is a number. The serial number is needed for
generating a reply message. EVENT is a D-Bus event, see
`dbus-check-event'. This function raises a `dbus-error' signal
in case the event is not well formed.