Greetings from Oklahoma

It seems so simple, but over the years, I've come to learn the benefit of asking questions before breaking things. I've been trying to remove the shift knob from my Fulvia S1. If I had to guess, I want to believe it's a normal, right hand threaded knob. I've tried to turn it by hand, then with a strap wrench to protect the knob, but it's either too tight, or I don't understand how it's attached. There's a company here in the US that makes custom gear shift knobs, but I need to know how this one is attached before I can order one. The company makes a four speed pattern engraved with right hand down reverse as is the design of my car. I suppose some might believe the plain gear knob is more elegant. That's probably true, until I have to let someone else drive my car - Yikes. Anyway, it's something I'd like to try. If anyone knows the thread size (if it is threaded), that would be wonderful. As always, any help at all is appreciated.

Thanks again,