Ha.... on Schwinn stuff, you'll find that they sold Sidewinder frames made in 1980 as '82s.... and in 1979, Schwinn had some leftover Super Sport f/b frames sitting around their warehouses, so they built'm up and re-labeled them as "Sport Limited" completes. These were the fillet-brazed 4130 Supersport frames with the American BB shell, but Schwinn ran an adaptor and a 3 piece, from the factory, which is weird. These were all red bikes, apparently. But, that'd be a case of frames built in/before 1974 being sold as 1979s, under a different name than they'd originally been manufactured for.
So, yeah, the serial will tell you how old the frame is, but it won't necessarily tell you the model year it sold as. But, for the more popular bikes and the older bikes, it'll most likely be really close.