Educate me: Why is this bracket attached to the AC controller, what trucks had them?

[DeadheadNM]

Here's an AC controller I found loose in the back of a Suburban at a salvage yard (together with most/all of the parts for a 72 burb rear AC setup). Why is the bracket in the photo attached to the AC controller? That on my K20 never had this bracket - it's just bolted to the dash like a heater controller. Also, the screws that affix this bracket to the AC controller are coarsely threaded like those for a heater controller but unlike the fine thread screws that attach my K20 AC controller.

What trucks had these and why not skip the bracket all together?? Is it for a Blazer/Jimmy to attached the underdash speaker? How is YOUR AC controller attached?