With the round, non-inset battery plate, it should be 1980-81. The serial number would have been stamped in the end of the fingerboard, but on a fretless, they likely would have left it off. In the electronics compartment, it could be stamped into the core wood. Does it have LEDs on the side of the fingerboard? If not, then it has a replaced fingerboard, and that may be why it has no serial number visible.