A lot of manufacturers now uniquely mark every item allowing the user to differentiate between identical products. Some of these individual serial numbers also give more pertinent information* such as the exact date of manufacture and even the person who inspected the product after manufacture.
Petzl have produced a very good guide on the care and maintenance of their equipment. This guide also provides information on the marking of their equipment (pages 4, 6, 7, 8 & 10): Petzl Kit Guide
S-Tec have produced a guide for marking of the S-Tec Duck: S-Tec Duck Engraving Guide
*In the case of Petzl products the serial numbers they now use look like: 11045FM7461 with the first two numbers (11) corresponds to the year of manufacture I.e. 2011, the next three numbers (045) corresponds to the day of manufacture in that year I.e. the forty fifth day of 2011, the two letters (FM) is the code of the person who inspected the product and the final four numbers (7461) is the individual product number.