Good afternoon. Looking at the attached picture you will see a return duct inside of a 2X4 studded wall. It sticks out ever so slightly, about 1/4″ past the wall studs. They have brackets to make them flush with the studs so you don’t have to push on the drywall. I’ve searched Supply house and a few other HVAC sites. I can’t identify what they are referred as. If you have a description and vendor, or part number, please share.
Thank you