Perfex Series 155 Magic Dial Thermostat
Here are photos of a circa-1944 Perfex Series 155 Magic Dial Thermostat, as well as the instruction manual.
Hoffman Vapor Heating System with Differential Loop Patents
Here are patents from 1922 for the Hoffman Specialty Company for their vapor heating system with differential loops. Thanks to Steve Pajek for finding and sharing these.
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