All authentic Stündenglass unit boxes have a sticker with a scratch-off authentication code on the back of the box, along with the serial number. The same serial number is etched into the body of the unit. These numbers are used to validate and to register your unit at https://stundenglass.com/register Items lacking these numbers were not produced by us. If the serial number you are trying to enter cannot be registered, you may own a counterfeit unit. Please contact us if you believe this to be incorrect.