About thirteen minutes into the above video, the YouTuber shows a screenshot from the Corning Museum of Glass's website...
...that asks whether the PYREX stamp in all caps definitely tells that the PYREX item was made with borosilicate glass - something I'd been told was a definitive tell.
According to the Corning Museum website, "the change from upper to lower case signified a re-branding of the trademark Pyrex® in the late 1970s but is not a conclusive way to determine, historically, what type of glass formulation the product is made from."
So that's apparently out the window.
The video does go on to explore the various other ways of differentiating borosilicate from soda lime pyrex: the blue tint, visibility in mineral oil, even a not-very-scientific drop test onto a patio block.
In the end, the best that the host came up with was to look for a made in France mark. Apparently the French know to use only borosilicate glass.
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