73 Gold Duster 198 slant 6

I have a one owner car sence 1973.
The car has a 198 slant 6 with the snake skin landau top.
I did some research and supposedly the car wasn't suppose to have the snake skin top. That was reserved for the 318 V8 and the 225 skant 6.
Was this option on my car a mistake from the factory or a special order from the dealer?im putting together a information brochure for car shows. Any information will be greatly appreciated.