Peculiar. Imperial inches were 25.4mm, so that isn't it. Could be they just slipped when writing the original specs, I suppose. "5.4" is just once key away from "6.5", after all. But you'd think someone would catch that, at the same time. Ah, well.

And, calibre 4 I would think equates to 4-gauge, which is about 26.72mm. What with the mentioned Brit tendency to "eyeball it", as it were...

...!
Stoke Mandeville, Esq & The Victorian Ballsmiths
"Nobody Want Verdigris-Covered Balls!"