I don't think that a 1938 chart was ever intended to be used for metric (in the UK), other then it had the drill bit size equivalence (if available).
I was just commenting on what my observation was, I think Olderdan does antique motorcycle restorations (I recall some past post, maybe not him), and here these non SAE/SI "standards" are his repair source sizes.
This chart even has the wire gauge sizes, even into the 'ott' nomenclature. We know how useful that system is.

I'm with you Marv, on the simplicity of metric threads.
But any measurement system we have, has no absolute base reference. A millimeter, or inch are just random chosen lengths, and turned into a human standard.