Sheet Metal Workers' International Association
The Sheet Metal Workers' International Association (SMWIA) was a trade union of skilled sheet metal workers. Such workers perform architectural sheet metal work, fabricate and install heating and air conditioning work, shipbuilding, appliance construction, heater and boiler construction, precision and specialty parts manufacture, and a variety of other jobs involving sheet metal.
Source: Wikipedia — Sheet Metal Workers' International Association (CC BY-SA 4.0)