Every watch has its own serial and model reference numbers, which identify specific information about the timepiece. The model reference number of a watch, which may be numeric or alphanumeric, indicates a particular model of watch (for example: Daytona 16520 Rolex, Patek Philippe Nautilus 5712R, Cartier Roadster Chrono W62019X6). Omega watches have unique eight-digit serial numbers that identify the pieces in Omega’s database. Vintage watches have the serial number engraved on the inside of the watch back. Modern watches have a serial number engraved on the exterior of the watch. It is also on the guarantee card that comes with the watch. Tissot serial numbers by year. The very early Tissot watches had the serial number engraved on the back site of the dial. Later on the serial numbers were made easier to locate. On most vintag Tissot watches you will be able to find the serial number engraved into the watch movement.