the case serial number tell us exactly the date when the watch case was made, but some watches were assembled later in older cases, special in 1946 after the world war they had produced to much cases, same happend at Breitling. So Heuer assembled later watches in much older cases. For example the first Autavias used cases from old tri-compax watches of the 1940th, only made a new dial and new becel and ready was the new Autavia from old cases. That is also the reason why their exist no Autavia between serial number 59+++ and 81+++, the old cases from the 1940th were used for the 1. and 2. edition Autavia, the case for the 3. edition was made new in 1964.