Can someone identify this HVAC duct component?
These are just crude supply registers to provide heat/AC to the basement. They could have been installed with the regular installation or added at a later date. There should be register covers somewhere with a screw in the middle that when attached to the vent controls the opening of the vent. We had them all over our unfinished basement. When we had it finished. we utilized them with the drop ceiling.
You can replace them with straight ductwork but you'll need supply registers somewhere else.