[Bruce Lancaster]

You really cannot. You can tell sequence of engine build, so one of your cars is early and the other 2 or 3 months into the production year...that allows reasonable guesswork.
Glass marks and the stamps that are sometimes on backs of instruments are additional clues, but as with engine, how do you tell how long it took a piece of glass to reach assembly?
I don't know whether Mercs were produced centrally as a special somewhat limited model or whether the parts were sent out all over the place to regional assembly plants like Fords.
New parts could likely hit assembly in Detroit same or next day, or much later at distant plants.
I think engine build can be dated by Rouge records, so your number can be dated but probably not when it actually went into a car.