'54/55/56/57, Fomoco offered one of two heater control systems in full-size cars. The cheapest of the two utilizes a vacuum water valve on the intake manifold with a bimetallic vacuum thermostat control valve located in the heater plenum adjacent to the heater core housing.
The second, and IMO the finest is the system that utilizes a capillary-tube thermostat water control valve mounted on the inside of the firewall above the heater plenum (with it's capillary tube extending down into the heater plenum).
Both of those systems are fresh air heating systems and will work either with the heater blower motor on or off when the "Magic-Aire" heater control knobs are in the correct positions (whereby the right airduct is opened by the mechanical cable connected to the "heat" lever on the controls).
There was a third and distinctly different system used for trucks and also used in some southern cars (most of which had no heating system from the factory), but if a customer requested his new car had heating, the factory would ship a F-100 recirculating heater to the dealer and they would install it at the local dealer. This was pretty rare, but I have seen a few of these. When used with a car, the recirculating heater was totally divorced from the fresh-air intake and relied completely upon air inside the car.
None of these had an engine compartment heater control valve.
There is a good illustration of the heater control system for the vacuum type on page 266 of the '56 Ford car shop manual.
Once upon a time, I owned a '57 Custom 300 with the capillary type control system which was identical to the capillary system in my '55 Fairlane.
