Just the opposite. Blowers are defeated until the coolant entering the heater core reaches xxx-temperature. This way, if heating is called for, there’s no blast of cold air into the cabin. The blowers come on when the coolant is hot enough to actually give some warmth. The trigger point is about 40ºC I think.
Disclosure: It’s 4:20AM and I’m just two sips into my first cuppa coffee
I can’t think of any possible difference except the trigger point. I’m inclined to agree that either would work. The 2121 switch was used on facelift models; the 763 on pre-facelift, as I recall.
The MKIII system went thru several minor changes over the years. This is one of them. I think it’s discussed in The Book.