One problem which indirectly refers to your watch is the fact that the movement and the case are manufactured separately.
In some cases(pun unintentional) the difference in year of production may be not years but decades. It's less so with modern watches, but was fairly common in the past.
I think you have to accept the year of manufacture as that for the movement. Case and any addition even if manufactured the same year probably have secondary importance. That would be my conjecture, anyway.