The first number shows you the Rolex model or which collection it belongs to. For example, a luxury Obsidian model, often worn by the rich and famous in the movies, would start with 182. The first part determines the total length of the reference number. Earlier watches used only two digits to describe the model, size, and movement type, resulting in a four-digit reference number. More recently, the first part of the reference number has been expanded to four digits, making the total length of the reference number six digits. 
For more than a decade (since 2009), Rolex serial numbers have been generated entirely at random. While Rolex dealers can tell from the serial number which year your watch was made, owners can no longer tell from the serial number alone.
There is an interesting historical exception to the idea that the serial number is a unique identifier. In 1954, Rolex ran out of serial numbers! Until then, the watches counted serial numbers (watch with serial number 7500 followed by watches with 750001). When they reached 999999, they started counting again from 010000. For this reason, two old Rolex watches may have the same serial number.
In addition to the two numbers on the face, Rolex watches also have a code on the back of the clasp, known as the clasp code or bracelet code. Indicates the year and month in which we created the clip. It is not unusual for the year of the clasp to be different from the year of the watch, so don’t worry if you find this. However, if the clip has been replaced, it affects the watch’s value, so this is something to keep in mind.
The serial number is on the inner ring under the glass/crystal at 6 o’clock. The reference number is on the outside of the case at midnight. Both are listed on the Rolex “papers” (paper or plastic). The reference number is based on the watch’s model, material, bezel type, and possibly other information. For instance, a look with a reference number starting with 140 is a Rolex Submariner.
Each Rolex is a unique treasure, and the serial number is a unique identifier. It does not tell you anything in particular, although, for older watches, the serial number may tell you the year of manufacture.
