We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember Stephen
saying something like:

Varies from maker to maker.

I'm surprised by this, since the pipes they fit are standard sizes, I
assumed the fittings would be standard sizes too.

Hah! It's plumbing we're talking about here. We should count ourselves
lucky that most compression nuts are a fairly standard size (note I said
'fairly', not consistently), so the centre hex portion of a joint is
whatever the maker wants it to be.