781 _is_ too low to be an '81. #781 would have been born in late summer, 1977.
Also, the earliest Spoiler I show is an '82: 82S2307, which was born either late September or early October, 1982. So the 1981 production date is suspect, as well.
David Grego (BigBass) from this forum owned the instrument in the past; he may have some info on the serial number and/or production date.