reverse thermostatic valves?

jumper

The opposite of what car engine cooling systems had.
These open only when return water gets cool enough.
In this way delta T is maintained for each circuit.
Were such devices used in the fifties?

Bill_17

These are typically called return temperature limiters, they are definitely available for single loop radiant circuits (small loads) and in larger sizes they have other names, most then have remote sensors attached to the valve actuator via a capillary tube.

hot_rod

Probably not available in the 50's. The Caleffi ThermoSetter, a thermal balancing valve, opens as temperature drops, to maintain a setpoint temperature.