M10x1.5 it has to be. An M10x1.5 would be slightly larger than a SAE 3/8" so would explain why a 3/8" might screw into the hole.
That said, they are very close, the 3/8" bolt is about 0.019" smaller and the pitch's are very close so the 3/8 -16 bolt will thread into the 10mm x 1.5 hole.
The use of a 3/8-16 bolt comes out to be approximately .024" to small ( about .012" on a side-l ess than a 64th").
To much wiggle might not be felt with grit and dirt working upside down and backwards.
The engineered Clarence for the threads to do what the deal was designed to do will be wrong.
It also has to do with how many threads are engaged in the application.
Wrong bolt in the threaded hole will fail something one way or another.
I posted all the information with the numbers and where to buy them but I can find that post now, don't know how. It was really good. I lost it in SS someplace in the last year or so.
Hey, ........... I found a metric thread in my Deuce, was I surprised!