Okay... my lead might have washed up. They "can't find" the one they had listed for sale, but are still looking at this time.
So, any other leads would be greatly appreciated! I did try to search for "Private jarhead" on here, but that user name doesn't come up.
As for part number(s), here you go:
NSN 5810-01-043-4643
SC-D-446186 (Left Mounting Plate)
SC-D-446187 (Right Mounting Plate)
SC-D-691342 (Large Mounting Plate)
At this point, I might consider borrowing one from someone as well if they could part with it. This way I could take measurements from all three components and make an accurate CAD File. I do have a friend at a fabrication business that could replicate it for me/us and probably even spray it the correct OD Green since they also produce smaller components for military projects regularly. Just a thought if I can't find a lead on one.
All though TB 11-2300-478-30-1 states the following:
"(1) Transition installation - All existing COMSEC Vinson security equipment will be transferred from the M151/M561 to the HMMWV except for the Support Assembly SC-D-691342, NSN 5810-01-043-4643."
This arrangement would work better for me when combining the radio installation with the VIC-1 Intercom System in my truck. CARNAC also confirmed that he did see them installed in this manner in the field as well. This way I can utilize the Intercom mounting plate as it should be, vertical, and them relocate the two speakers I have to alternate locations.