I'm almost sure that bass is newer than 1991, because it has the flip out plastic battery holder. It appears to have an unfinished neck too, but I can't really tell from the photos. You should head over to the Ernie Ball forum with the serial number a get the correct manufacture date. Good luck with the sale.
According to Gav's site, the metal cover to plastic flip out transition was 1996, but your neck plate looks like a pre 1998, So I'd say your bass is late 1996 to late 1998. See:
http://myweb.ecomplanet.com/TOWN8019...ompage0029.htm