Rules of thumb: Without known anything about the VIN structure, which eventually became standardized between manufacturers during the '80s.

Volvo, like all the other car manufacturers, release their "new" models during late September. Those cars are generally produced during the early part of that month. So, by using the PRODUCTION date, one can assume that any vehicle manufactured during Sept, Oct, Nov, Dec was built as the following chronological year model.

Due to different levels of change between years, there is no rule for judging how long the factory must close at year's end to gear up for the next year's models.

The other guide, for late '70s models and newer, is to see what model year the emissions label states in the engine compartment.