Fender bought a whole pile of serial number decals in 1978. It took them a long time to use them up. Heck, the pots only tell you the oldest it could be, not when it was actually built because they could have a large stockpile of them too. And between 1983 and 1986, they were making instruments with strange combinations of features as they used up the parts in stock.
The history is relevant here. CBS hired Bill Schultz around 1980 or '81 to run the company and bring it to where it should be. But as time moved on, CBS decided to divest themselves of almost everything not related to the core business of communication. So the whole Fender/Rogers/Rhodes went up for sale (as did Gulbransen organs, Selmer wind instruments, Leslie, etc.). Schultz headed a group of investors who bought Fender from CBS in 1985. They didn't get any real estate, only rights, patents, WIP (works in process), and some machinery. So they had to build a new factory, tool up to build things, and not loose too much money during the transition. That's what "Fullerton" means. It's only in reference to the Vintage Series instruments because that's about the only thing that was ever built in both the Corona factory and the original Fullerton factory. MAYBE the first American Standard Strats were built in Fullterton, but I don't recall the year those came out.
So, they used whatever stuff they had to make instruments. And they didn't care about serial numbers as dating information. Bottom line is that the serial number is only an identification thing for inventory, sales, warranty, and accounting reasons. Beyond that if they keep good records and use the numbers in a logical and sequential manner you can use it to help date an instrument's production. But most guitar makers are in the business of selling new instruments, not keeping track of old ones. So, combine the serial number, the features on the instrument, the pot codes, and any dating/identification marks on the bass itself to determine when it was made. But it's highly unlikely that your bass has a neck that was made a full three years before the rest of the bass. It's just that the S8 decal was three years old. Best way is still to take the neck off and look for production information on the heel.
John