C42 Deluxe Air Heater

[dagnabbitt]

In my 67 GMC I noticed this C42 option on the SPID, wondering if anyone can tell me exactly what the difference is between this option and the standard equipment. There is nothing obvious when sitting in the cab, no extra settings etc on the controls.

A search of the forums tells me that some people think that in 1967 a defrost heater was still novel enough that it warranted an inclusion on the SPID, but I am skeptical of that. If it is simply a heavy duty version of the standard equipment I wonder if it is a bigger/better fan, heater core, or what.