I believe my 77 F150 with factory AC calls for a vacuum switch P/N D5TZ-19B888-A. I may have inadvertently purchased a switch P/N D8TZ-19B888-A.
The D5TZ is listed for up to a 77 Truck and the D8TZ is listed for a 78-79 truck.

Will the D8TZ fit my 77 truck? If so, what is the difference between the two?

 

The parts catalog doesn't specify what, if any, differences there are between the switches. All it says is the D5TZ switch is for 73-up F100-250 with integral A/C or built in A/C (dealer installed) with Hi-Lo vent.
It says the D8TZ valve is for 78-up F100-350 w/ deluxe heater vent.
Here are pictures of the two, I can't see any difference.

 

I agree with you. That is why I asked you gurus here on FTE. The valve will be here within the next day or so. I'll take the dash apart and let you all know. Maybe it was just a minor change that didn't change fit form or function?

 

Unless one is a pin actuated and one is a loop?