shifter ball thread size?

The thread size is exactly this .. M12 X 1.75 6h, and not just 12MM like some people will tell you.

I'm a machinist, and I had to custom fit a B&M T-handle to my car. I couldn't find the metric version, so I just figured it out for myself. I work with threads every day of my life, so it was pretty easy!
 
12MM is much larger than 3/8. In decimal form 12MM is .4724, and 3/8 is .375. 16 threads per inch differs by about 1.5 threads per inch as well. 1.75 pitch is 14.51 threads per inch, so you're gonna have to buy another one buddy! :(