Its one of these I'm wrestling with
http://www.ebay.com/itm/Vintage-WPL-...item27f5fdf266
The spindle for turning to change the setting, seemingly now temperature
and not current.
Thermos as in themos flask?
I decided it was not working and after taking it a apart and checking
its action it seems ok. I suspect someone removed a thermal bridge to
the metal plate, and it should be functioning as a thermostat proper,
feeding back a fraction of the heat. With a magnifier there is a trace
of where a couple of screws were in place.
So surface unit ? to add to the growing list of non-generic names for them