Actually, your DA-15 <-> DE-9
missing cable and Pivot''s configuration could help out substantially in the 8 header pin <-> DA-15 <-> Integrated HD-15 Adapter cable pinout project for the, readily available, Radius Color Pivot II/IIsi Cards.
With the Pivot powered down:
Hook up a DE-9 Serial Cable to your Pivot and meter the pins. to the D-Sub shroud along with second and third, obvious ground points (in this scenario the ground lug on the three prong power plug comes to mind.) to let us know which are the ground lines.
Pivot the monitor to test if any of the "ground/signal" pins' status changes with the rotation.
With the Pivot powered up:
Repeat tests above with the exception of the cable ground lug.
If one of our electron-pushers can noodle out a test for the remaining lines that change with the Pivot's orientation when the monitor is powered up, things will be simplified quite a bit.
@oldappleguy, if you can do these same tests on your identical DE-9 <-> DE-9 cable/Pivot and especially the DA-9 <-> HD-15 cable/Pivot, we'll have an interesting table of results to work from, especially so as the 8 header pin <-> DA-15 Cable for the SE/30/IIsi in my possession will give further clues as to what's going on within the Card <-> Cable signal interface.
Have you got NuBus or PDS Cards to go with your Cables/Pivots?
I'll post more testing procedures as they come to mind, but if one of our boffins chime in to help, maybe what's left of my mind won't frazz . . .