It's a supply plenum or duct plenum created by CEN Fabrications. The plenum is a box‑shaped sheet metal piece used in HVAC systems to connect an air handler, furnace, or coil outlet to the ductwork, distributing conditioned air into the ducts.

The dimensions 16.5″ width × 17″ height × 24″ depth indicate a relatively compact plenum, probably used where space is tight (e.g. compact utility closet or small air handler).

Likely uninsulated, unless otherwise specified, since many plenums are built without insulation unless required (for noise, thermal control, condensation prevention).

Probably involves standard galvanized steel (galv. sheet metal) construction by CEN, with seams, flanges or collars for connection to ducts.