Quoted from joetechbob:Was able to turn it into both sae and metric. Metric seemed like a better/smoother fit though.
Forgot to grab a pic of the washer.
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3/8 -16 bolt will also thread into a 10mm x 1.5 nut, but the 10 x 1.5 bolt will not thread into a 3/8 nut.
Based on Joetechbobs checks, I would go with 3/8-16 bolt and get some 3/8 large flange fender washers for it.