46 CFR § 131.899 - Fire dampers.

Each fire damper installed within the boundary of a space protected by a fixed fire extinguishing system must be fitted with an indicator showing whether the damper is open or closed and must be marked with red letters at least 13 millimeters ( 1/2-inch) high stating “FIRE DAMPER” and, as otherwise appropriate, identifying the space served by the fire damper.