Looks like Packard used a variety of terms for a circuit breaker. Thermostatic switch, thermostatic overload relay are used in 38 manuals. On 39 switch, it appears to only protect the headlights.
Since switches are interchangeable though, if needed maybe even a later 39 switch could have been installed in service on earlier cars.
Looking at the wiring diagram, I would agree with flackmaster. Very important distribution point and would almost need the proper switch to avoid a butchered rewire job.
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