Over the years, no one I have talked to at AKG seems to have a grip on serial number logic or correlation with dates. All we know is that, usually, lower serial numbers indicate an earlier manufacture. But neither the sequential nature of these numbers has ever been confirmed, nor has there supposedly (and incredibly) ever been any documentation kept at AKG as to when a particular mic with a specific serial number was made or left the factory. To me, that's pretty hard to swallow, given that companies need to inventory stock, at least once a year, for tax purposes.
That being said, there are plenty of build features of, say, a C12 that make identifying its year of manufacture pretty easy.