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

Funny, I was at dealership yesterday and parts guy punched in both 67 and 68 numbers and yes to superceded numbers. BUT parts guy said 68 superceded back to 67 number?? WTH??
Did you get anywhere figuring out whether '67 switch was different than '68? and whether there was a superceded p/n number?