It's pretty easy to find the correct Heater for the
1931 Chevrolet. It was introduced in the Chevrolet Service Bulleting
in September 1931, thus barely making it a '31 accessory. But having
the original dashboard on-and-off switch is pretty rare.
Here are various shots of the correct heater
switch. Note that it has a wire-wound resistor underneath it.
This allows the switch to have a "high" and "low"
speed on the fan. There are three switch positions with the
"off" position being in the middle.