Looks like it adjust the water flow to the heater. Do you have a rear floor heater?
I was looking at a bus the other day that had a valve like that right under the dash
And it controlled the under seat heater in the rear.
Could be that your thermostat is ok but has been changed sometime in the past.
It might be a 180 degree and should be a 210, or the other way around. there are always ???? When you buy used..
I always try to document what I do to a vehicle like a maintance record to keep in the glove box for me and to pass on, makes my vehicles a lot easier to sale, and gives them better trade in value.
Don and Ellen
Plus one fuzzy faced kid (Poopcee)