Problem solved , my error.
First I upgraded Avitab on my older XP11 PC from v051 to v063. Tested OK.
I compared IXEG files from my old XP11 PC to my new XP12 PC.
I noticed Avitab.json was missing from my IXEG737 Classic Plus folder on my new XP12 PC.
I copied the Avitab.json from my old Xplane11 PC to IXEG737 Classic Plus folder on my new XP12 PC (with Avitab v063) and that fixed the problem.
This missing file was most likely my accidental deletion when following file backup / install instructions on my first AVITAB download/install.
This Avitab install did not work so I removed it.
The readme for this bad install :
Make sure to backup your B733.acf and B733_cockpit.obj
Copy the contents inside the compressed file to the IXEG 737 Classic folder, overwriting the existing files.
So now I assume I am ok with the Avitab.json I am currently using without a need to download a more current one?