Some cars (some Fiats, I think) have wheel bolts. but one of the two small bolts that hold the disc in place has a pointy end on. There are four small holes between the wheel bolt holes. It's so you can easily get the wheel hanging there while you start the wheel bolts. I'd guess the Lincoln is the same, and as the top line model, it was worth the expense to provide the helper stud. Higher volume and cheaper Fords and Mercs wouldn't warrant the expense.
Purely guessing here, but maybe (as the more expensive model), the pointy stud is aligned with the wheel valve and balanced as an assembly. Better balance for the more expensive model. Again, just guessing, but that could possibly explain the use only on the front.
Thanks for the original post, i never knew the Lincolns were like that.
Mart.