The Gretsch-GEAR database exists to get it right: to document the actual serial numbers, to determine once and for all what was built and when, and to harness the collective knowledge of thousands of Gretsch fans to put the pieces together.
If you'd like to learn more about Gretsch serial numbers, we highly recommend Ed Ball's "Gretsch 6120: The History of a Legendary Guitar." Ball is one of the key researchers who have determined the actual dates, years and features Gretsch serial numbers correspond to.
When Fender took over production in January 2003 the serial number scheme changed again, and Fender-era serial numbers have a two-character originating factory code, followed by year and month, and finally a sequential ordering number.
For example, serial number JT03074463 would break down as: Of those, JT is by far the most common on the pro-line instruments.
A great resource to date a vintage guitar amp is to check out the Blue Book Of Guitar Amplifiers.