With some brands & models (including Guild), spec changes have taken place in mid year. Some folks like pinning down exactly when those changes took place.
For me, as a researcher, it's this. It's pretty easy pinning down year of manufacturer and, in general, that's fine. Knowing when certain features changed is more important. Sometimes that works out well, but having done a lot (probably too much) research on Ampeg amps for the Ampeg book and Fender amps for my series of articles, I can say that no matter what general dates are determined, there are always exceptions. Hans has showed us many examples of exceptions with Guild. And the exceptions are what make us crazy.
Then there is year of manufacture versus "model year". Gretsch was, and sometimes still is, infamous for this. All those 6120s that were made in late 1959, but were sold in 1960 as "new and improved" for the 1960 model year. So what are they - 1959 or 1960? Dealers sell them as '59s because there is a premium for that year. It's just like automobiles, too. My car has a tag that clearly states it was made in October 2015 yet we call it a 2016 due to its features not its completion date.
Another example - my 2012 Martin has a 2011 serial number, but it was a limited edition for 2012. The serial number was probably assigned and put into the work flow system late in the year although construction may or may not have started in 2011. I took delivery of the guitar at the Martin factory in May 2012 so it likely has an April or May 2012 completion date. Martin calls it a 2012, but there's that dang 2011 serial number!
A fun thing G&L did beginning in 2017 that, theoretically, should have made all this a bit easier was changing to a date encoded serial (Year/Month/Nth guitar made that month) and providing a "Build Sheet" with each guitar. This showed the specifications for the guitar or bass along with the completion date. That worked pretty well for most of 2017 until the wheels came off. I have many examples of guitars with serial numbers from 2017 through 2019 that have completion dates months to years later than the date encoded serial number suggests. Once again, "exceptions exist".
In the end, I've learned not to obsess too much about pinning down the exact completion date of any guitar or amp. Knowing the year is good enough though playing the "year made versus model year game" can be frustrating just as trying to pin down when features changed will always be a challenge and never exact due to "exceptions exist".
