Nice write up and pictures.

Did your instructions tell you to use Dextron ATF?

I was always told not to use that.

Searching the web I found this write up. Which is what I have always used.

The overdrive is shipped without transmission fluid in the unit. Remove the 13mm fill
plug on the back right side of the overdrive and add approximately 26 ounces of synthetic
manual transmission fluid (GM part #12346190 or Mopar #0487-4459). Fill to the
bottom threads of the plug. Gear Vendors specifically recommends GM# 12346190, or
Mopar #0487-4459, and it should not be substituted.

Gear Vendors previously recommended Dexron II automatic transmission fluid for its
overdrive units. It now recommends the GM synthetic manual transmission fluid for its
overdrive units due to improved shift characteristics, and less thinning at temperature
than Dexron automatic transmission fluid.

GV's website says - Fluid type - GM#1052271 petroleum based or GM#12346190 synthetic.