Here are photos of a circa-1944 Perfex Series 155 Magic Dial Thermostat, as well as the instruction manual.
Hoffman Venting Valves for Every Purpose (1918)
This book, Hoffman Venting Valves for Every Purpose, was from the Hoffman Specialty Co. in 1918. It includes a lot of information about Hoffman valves for heating systems. You'll also find a discussion of Hoffman's Equalizing Loop, which came before their Differential Loop. Thanks to Frank "Steamhead" Wilsey for finding and sharing this document with us.
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Perfex Series 155 Magic Dial Thermostat
The American Society of Heating and Ventilating Engineers Guide (1929) - Hot Water Heating Systems and Piping
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