Center A/C Vent. Can't figure out what it attaches to

[Kirk52]

I am working on a '70 Chevy C-20 and am in the process of putting in a retro radio and new dash speaker. I removed the previous radio, old single dash speaker and the center AC vent tube. That was several weeks ago. I ordered the new radio and dash speaker but now I can't remember what the center AC vent tube attached to. I plugged the driver's side opening on the center vent and am ready to "re-install" it but now I don't see where the studs on the back of the tube are supposed to mount.
Any help would be appreciated but go easy, this is my first post...