It is required by law to have your glass stamped with the CE mark (government stamp), for any glasses that are being used in licensed establishments, such as bars, pubs, restaurants, beer festivals etc. This ensures that you are serving the correct measured amount and complies with laws on the service of alcohol.
The CE measurement mark can be to the brim or a line marked on the vessel. The ‘M15’ states the year in which the CE mark was applied, for example we are now on ‘M17’ for 2017. The 4 digit number is the registration number for the government body that verified the CE marking. The image shows the CE marking for a 2015 glass, with lines printed at two third pint, half pint, and third pint for correct measurements.
If you are using your glass as a measuring tool for serving alcohol (beers/ cider/ lager) you need a CE mark. If you are using the product at home or using an alternative measurement tool (that’s approved) then the glass doesn’t need a CE mark.