I gather from what your saying, all of the 1947-54 Chevy and GMC trucks have valves with a capillary control tube, like that shown in Chevy Duty’s catalog, is that right???
The valve with the capillary control tube was used on all 47-54 Air-Flow (Deluxe) heaters. No such valve was used on the more common recirculating heaters.
Here is a picture of the two types of heaters. (The 54/55 Air-Flow heater was slightly different from the earlier Air-Flow heaters).
Both types of heaters had a
heater fan speed-control switch. The "purple twist knob" that you refer-to is, I presume, an after-market fan control switch
like this one (can you post a photo of the switch)?
Tim
