So to determine when your watch was manufactured, you will need to find where your serial number fits within the range of numbers.Let's say you have a Waltham watch with serial number 21,607,210 as shown in the photo below.The date of market introduction may vary however, so expect to add 6 to 18 months.
The serial number of your watch is engraved on the case at the 6 o'clock end between the lugs.
You will need to remove the bracelet/strap to see it.
Data may not be accurate for Limited Edition Speedmasters and Speedmasters with cal. This serial number data item not given in Hartmann’s data but empirically derived from a consensus of users in various internet Omega collector forums.
Seems to occur most frequently with Omega Speedmaster Mark II models.
BELOW: A close-up view of the same watch, you can clearly see the Serial Number "F982176", engraved into the case at the 6 o'clock position, with "ROLEX" engraved repetitiously around the dial in a circular fashion.
In 1927, Rolex began issuing every Rolex Oyster case a unique serial number to distinguish them from one another.
While it was originally believed that this resequence started over at 100,000, or even 20,000, but evidence suggests that it actually restarted at 10,000 or possibly lower.
Around this same time, Rolex had initiated another dating system--They stamped the inside of the case back with a code consisting of a Roman numeral and two numbers.
Some American watch brands did not use a consistent series of serial numbers, but most of the big manufacturers did.
Most vintage Swiss pocket watches did NOT have serial numbers and can't be dated by this method.
However, they soon discovered that it had some serious problems in addition to its much-touted advantages.