Why do 68 & 69 dart iginition switches have different # ground plug ins?

I guess you didn't read what I posted in the other thread. WHERE did you get this diagram? I posted one out of a 68 Plymouth manual, the Barracuda inst panel, that I'd downloaded from MyMopar

There is no switch that needs a ground, unless it is some sort of lighting, or anti theft. GM used to ground the solenoid circuit on shutoff to prevent jumpering the solenoid

This

68IGN.jpg