As far as I know the Ford PN was the same for the front to back so the threads size would be the same. The purpose I believe of the intregral washer (not two different pieces...all part of the nut) is to spread the clamping force equally over the bearing surfaces (for the fronts) or the hub surface (in the rear) which is part of the reason the nut torque is 250 Ft Lbs...you are pre-loading the bearing pack inside the front hub.
The nut has a series of plates inside its cage that deform as tightened on the spindle or half shaft end. It is part of the deisgn to have them stay tight....kinda like self locking. They are supposedly a single use design.
The way I read the Rockauto info was the fronts came in packages of 5 (you had to buy them as a pckage even though they quoted the price as the each price) and the rears came in packages of 2.