Technical 1936 Oldsmobile shift knob thread size

I am calling on the HAMB Brain Trust to set me straight.
I have a paid-up buyer for one of my classic-looking, one-of-a-kind shift knobs. He has a 1936 Oldsmobile F-36 and it seems that he's not sure how to determine the thread of the shift lever. (I'll double ask him to make sure it's the original trans. Assuming a restore, not a rod.)
Does anybody have the straight-skinny on the thread? Help a brother out.
Here's a pix of the knob ...... and a shameless plug for my hobby/biz.......

 

This is a useful item

 

Yea, thanks, my friend. I have some of those myself. Sadly, I occasionally have to deal with folks who can't tell a 1/4" thread from a 1/2" thread and don't even own a basic toolbox...... and they are a long way from my driveway. A lot of shift knob threads back in the 30s were 5/16", but again, some were fine and some were course. I can't afford to send one of everything and I hesitate to guess. I have to do a lot of hand-holding but sometimes I have to ask for help when I'm dealing with a clueless customer.

 

Can't help but I can give you a bump.

 

Tell him to buy a couple nuts at the hardware store and tell you which one fits.

 

Agree, assuming the customer doesn't get them mixed up on the way home

 

Throw away the nut that doesn't fit. Mail the good one to @The Shift Wizard .