NUT SIZE OEM DRIVER'S SIDE MIRROR

OK, I give up!

I installed a passenger side mirror on my 67 Barracuda, and it helps me with the passenger side blind spot trememdously.
But, getting to it, and tightening the nut on the end of the fixed post on the mirror was a PITA!

I did not make a mental note on the nut size on the end of the fixed post.
And, I bought a really nice OEM remote mirror for the driver's side for a great price that I want to install. As, mine was pretty scuzzy.

What is the nut size on the OEM driver's side remote mirror's fixed post?
Anybody??

Trial and error just isn't going to hack it.
And, I have looked all over the web for a diagram with no luck.