Many American Masters will either have a paper serial sticker or nothing. Typically you'll find serial numbers that indicate production the year or years before the guitar became officially available. Your serial number could be for a year long after your guitar was available.
Month, production number that month, year. They would no longer be restricted to using a specific plate in a specific year and any batch of plates would be good forever.
The first digit will be the last digit of the year, ie. Also there are many guitars whose necks have been replaced due to breaks or just bad necks. Was it still available somewhere else in the world where it still had a strong market and was still being sold. Some have actually had two numbered plates and some have had one numbered and one unnumbered plate.
Prestige - Exclusive Dating Agency
These might be in a different color, have different inlays, or whatever other spec made it different. In preparation for a model release the factory will produce as many as they believe they can immediately distribute so there are guitars available when the model is announced. Guitars with no serial can only be dated by spec to when that guitar is shown to be available. This is where you start seeing consecutive numbered plates on the remaining regular joint models that no longer indicate year.