It's the model of end link when the salesman originally ordered the bracelet, he'd know which sort of end link that was necessary to make the bracelet match up against the specific case. The bracelet can be used on a number of different watches, but the sit flush against the case, a certain end link is necessary.
As
@cristos71 says, it would be interesting to know what type of bracelet you have. If you open up the clasp, there should be some markings there - the four digit number is the bracelet model code
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