> In the UK, and I suspect elsewhere, pub glasses are manufactured to indicate their age

I think that is a TikTok 'story' which sounds reasonable and is spreading despite it being not true. Yes perhaps the year can be an indication of older glass and perhaps they have more wear, but that is down to use and care rather than just age and not what the year means.

The M mark indicates the year the tooling for that specific glass was certified and marked. Years ago the glasses were manufactured, then tested for capacity, and finally stamped to show they passed.

More recently the manufacturer, this is a major factor for plastic glasses which are moulded, has their process and tooling regularly certified and random samples regularly tested. This means they can be marked during manufacturing rather than an additional process.

This has a good overview of the markings on a UK drinks glass https://advancedmixology.com/blogs/art-of-mixology/what-does...

A bit of legislation from Trading Standards who police it. https://www.southwark.gov.uk/assets/attach/2503/Alcoholic-dr...

A bit of Government legislation about applying the mark. https://www.gov.uk/guidance/applying-a-crown-symbol-to-pint-...