I've posted a fuller reply, but they year on the M stamp is about certification of the tooling used to make the glass. This can be updated annually and so does

I've posted a fuller reply, but they year on the M stamp is about certification of the tooling used to make the glass. This can be updated annually and so does give a decent indication of year of manufacture, but it is there as part of the Weights & Measures legislation and not about obsolescence / "use by" etc