Too small for 1/4 BSP.
Non BSP pipe / fluid / air connection threads are a major nightmare for " ….. can fixit" folk. All sorts of weird and wonderful stuff out there.
At 0.430 measured outside diameter I'd plump for 7/16 UNF, which is 20 TPI and not uncommon for such duties.
Another possibility is 12 mm x 1, 12 mm x 1.25 or 12 mm x 1.5. Over the years I've needed taps for all three!
As a side note 7/16 UNF is often found in the cast alloy handles of Far East import air tools with a male BSP parallel adapter screwed in. A right pain when someone wants you to fit a QD air connector directly to the handle after wrecking the original getting it out. Super Godzilla level threadlocker-sealant being the norm.
Clive