Not real sure on that question. I think it had more to do with the assembly line...where the truck was produced and what line it was made on, too.
Not all the trucks had them. I do believe it was an option if such item was desired, but again like I said, if it was not asked for on the option list, the truck could have came out with them regardless. Just one of those little items that some buyers don't pay attention to I guess.