The Southwark 51F81410 / RB81410 is a straight ceiling box with flange designed for HVAC duct transition from a ceiling or plenum surface. It provides a mounting box and collar interface, enabling a round duct (in this case 10‑inch diameter) to connect into a ceiling or flat surface in a structured, sealed way. The box is built from steel and finished with hot‑dipped galvanization for corrosion resistance, making it suitable for indoor HVAC environments.

Its flange allows you to fasten or seal it against a ceiling surface or ductboard, while the collar section extends into the ductwork system to accept round duct. It is intended for a 10-in duct collar, so it serves as a junction point in ventilation systems.