As far as I know they are the same car, same body, same dash and speedo. I wonder what they think the difference is? Do they have old catalogs. They might be able to look both numbers up and compare them for length and style of ends.
If you know the length of the cable housing a place in California called "Red Line Gauge Works" can make you one. They will need to know what year transmission you are using.Or get the one for the Belvedere I. Both ends are screw on. I'll swear to you and stick to my guns that there is no difference between both the I and II.