It appears they're not metric and therefore I'm lost

Ideally I'd like to get some wingnuts but M5 are too small and M6 too big.

Does anyone know the correct size? Thanks.

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What's the thread major diameter? Likely a 10-24 if M6 is a little too big.

@jd74914 My vernier caliper says 0.206". Thank you.

A bit more research and with the help of an engineering friend we've found that the thread is UNC 12-24. My chances of finding a wingnut in that size in the UK are going to be slim.

Wow-didn't even consider that since almost no one in the US uses #12 screws either.

I've managed to find a source in the UK. They're made to order though so I'll just have to wait a couple of weeks. Fortunately they weren't expensive.

Problem is wing nuts will back off from vibration, that's why they come with nylocks.
I'd use some light duty loctite (purple, loctite 222) on the wingnuts so they won't get loose all the time.