On a side note, I really love the look of the earliest 601/602/604 upper receivers. The way the rebate on the upper lines up with the edge of the lower looks awesome, as does the angular "square forge" in front of the port door. Other than the green Cerakote, that picture at the top of this thread is really making me droooooool...
It's too bad that the "earliest" uppers (and almost all of the other parts) are almost unobtanium, because the earliest 604 is my favorite AR variant. They have a couple of useful improvements over the 602/602 but lack the bolt forward assist of the 603. They are "Clean and Mean". I'll bet even Mr. Stoner approved of the evolutionary modifications that led from ArmaLite's rifle to the Colt Model 604.
It would be tempting to buy that rifle and attempt to "rescue" the upper receiver. Maybe what the seller is calling "Cerakote" is actually something less tenacious, like Gunkote or even paint.