This is a 1952 ad for the engine-driven BN52 model Mobil Directo air raid siren, which used a 25HP Wisconsin air-cooled engine to drive both the chopper (the noisemaker) and two belt-driven pulleys to spin the siren around. Later BN44E models would switch the engine to an electric motor. It was one of the first rotational sirens, and would later redeveloped into the "Allertor" and "Penetrator" electric sirens.
Civil Defense contractors who work with municipalities to install systems of sirens to their desired spec. There are still companies that do this today such as W.S. Darley and Edwards. City civil defense officials would also look at these ads when deciding which company to go with for a siren system.