Stone guards mounting bolts

[kfet]

A couple of these were very worn. I thought they looked like M10s but the M10s won't fit. (The thread is very similar though!)

Does anyone know what kind of threads they are?

[kfet]

It appears that some of the bolts currently fitted are 3/8". I compared these to M10x1.5 and they are very similar (but not identical). Could it be that the originals were M10x1.5 and that someone has put in the 3/8's? Or is it possible that a Japanese car could have been fitted with imperial threads from the factory??