My mate Dave used to work at a glass cutter's in Derby twenty odd years ago.
He told me that thanks to The EU good old Brussels had levied a procedure and manual on their trade.
In short they were responsible for ensuring the correct type and gauge of glass was supplied, 'Elf & Safety don't you know.
As usual everyone else in The EU ignored it and we embraced it passionately. Which is why as soon as you ask to buy glass they ask you what it's for/how you're going to use it etc. If it's not bog standard like for a window or picture frame they can refer to a trade manual which indicates thickness and type.
I think 20 years on they've eased off a bit (sadly Brexit won't change anything) but at the time I was looking for a piece to cover the playfield of my 1930s Genco non electric type bagatelle sort of game.
They insisted that due to it being described as an amusement machine they
had a responsibility to the general public only to supply tempered safety glass and quoted me about £60 and a lead time of about a week.
I pointed out that window glass would do fine and the groove wouldn't accommodate the thickness they were insisting on.
It did no good.
As I said they seem to have slackened off a bit nowadays. However to be sure, if I buy glass now I describe thick window or picture frame as the use.
If it's metric and rattles around in the imperial gap I whittle a very small piece of wood into a wedge and push it between the glass and frame at the top where it's out of sight.