D9 18A1901. It has a transmission pressure gauge screwed into the top of the transmission filter cover. It doesn't work. Caterpillar gauge. None of my parts books for D9Ds or Es show it. Cat dealer is a no go as well.

Anyone have a part number for a transmission pressure gauge for an 18A?

That is most likely just a test gauge. If your tractor is operating properly you only need to remove it and put a proper plug in it's place.
I just bought a OMI for the D9 18A & 19A. I'll scan it for the ACMOC library. When I'm done I will make all my physical D9 manuals available to "paying" ACMOC members.
I like your polite attitude and manners. 
I also notice you have been a freebie "user" since May of 2018. 
This is my invitation to you to become a "paying" member and enjoy access to our library and our quarterly magazine.
Please help support our club,
Craig