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As to width, while 1/8" translates as 3 mm which is, at least, a round number, 3/32" translates as 2.38 mm, which seems completely random to me.
except that 1/8" equates to 3.17mm.
I know that well from looking for a specific Simplex TdF rear derailleur. They were often stamped with the number of gears and the chain width each version was designed for.
Big_Block-
I bow to your superior knowledge. So that makes the example better, since 1/8" was the original dimension and apparently it converts to a daft number that no one would have started off with.
I took the figure as '3' because that is the number stamped on TA 1/8" rings, but I expect your Simplex figure is more accurate. This shows how easily errors can get into a system.
I understand ( although this only hearsay ) that the reason for the inconsistent sizes of conventional head set races has arisen because of inaccuracy in converting from imperial to metric. That's to say, they were originally imperial, but when these parts were made in other countries the dimensions were metricated, but not in a good way!
clubman
Have you checked whether the chain rings you have (the Williams and the 5 pin for the Durax) are 1/8" or 3/32"? If they're 1/8" they won't work with the 5 speed block.
I should mention here, metric and imperial. I am definitely not a brexiteer, but I generally prefer imperial measures. This is especially the case when the subject to be measured was made to imperial measure. So if an English frame was made as, say, 21" I find it irritating to see it described as a metric size. Chains are a case in point since they are virtually all still 1/2" pitch because the standard was set in England. As to width, while 1/8" translates as 3 mm which is , at least, a round number, 3/32" translates as 2.38 mm, which seems completely random to me.
BTW I've been away for the past few days, and I'm off the internet (intentionally) when I'm at my other place. That's one reason why I go silent from time to time.