M9 nut for rear wheel axle?
It's not M9. M9 is never used on rears. When 3/8&quot; axle hardware sizes are expressed in metric, they're called 9.5mm.
What you have is almost certainly 3/8&quot;x26tpi. If you don't have a caliper and thread pitch gauge, carefully taking the measurements from the axle with a ruler works too. There is some nonzero chance it's M10x1, but not likely.
A single 3/8&quot;x26tpi locknut is something probably easiest attained by pulling off a dead wheel or hub. It's one of and maybe the most common sizes in the world. If you're squirreling through random axle hardware, distinguishing between 3/8x24 if all you're measuring is the nut with a thread pitch gauge is impractical, so again, measure the axle.