3 June 2013: Updated with additional pictures and information.
The McDonnell F4H Phantom was delivered with unique external tanks, a big 600-gallon centerline tank and somewhat smaller but also bespoke 370-gallon wing tanks. The two General Electric J79s enabled it to lift a heavy load but required a lot of fuel, particularly in afterburner.
The 600-gallon tank with the "long" pylon fairing (which suggests that there was a short one; I don't know what it looks like yet):
The first 370-gallon tanks were provided by McDonnell. They can be distinguished from the subsequent Sargent Fletcher tanks by 1) the continuous increase and then decrease in cross section and 2) the flange on both sides of the tank where the upper and lower halves were connected together.
I'm pretty sure that this tank was 240 inches long and about 26 inches in diameter based on scaling a McDonnell stores drawing provided by Mark Nankivil.
Strictly speaking, the pylon and McDonnell tank were separate items, unlike the later tank. I had wondered why there were no sway braces involved or gap between the pylon and the tank (there was a small gap between the pylon and the wing—see picture in the discussion of the later tank) until Rex Droste provided this illustration, which shows that the tank was bolted to the pylon, not simply hung from it:
Installation of the tank on the pylon was obviously not as simple as hanging it and connecting the fuel and pressurization lines...
As you can see from the above picture and the following one, the forward mounting flange of the pylon was not symmetrical.
The later tank was supplied by Sargent Fletcher. You will see statements to the contrary (the early tanks were Sargent Fletcher and the later tanks, provided by McDonnell) in print and on the web but I'm all but certain of this.
The later tank is distinguished by having a constant cross section for much of its length and only one flange, which was on the lower left side of the tank.
A view of the right side of the Sargent Fletcher tank, showing that there is no flange. (The line on the tank is a painted pin stripe.)
The shape of the aft lower side of the pylon had to be different due to the changed shape of the tank and in this case, the pylon and the tank were a single unit.
Craig Kaston provided the following picture of an installed Sargent Fletcher pylon/tank and noted that there was a gap between the pylon and the wing.
The McDonnell tanks were on the first operational F4Hs. I'm not sure when the changeover to the Sargent Fletcher tank was made, but I vaguely remember there being two different tanks while I was a flight test engineer for McDonnell in 1966. According to an illustration of Sargent Fletcher history, it began delivering F-4 tanks in the late 1960s, roughly corresponding to the changeover in production at McDonnell to the F-4J. Rex Droste reports that there are dated pictures of the McDonnell tanks being used as late as August 1974, at least on USMC F-4s. I'm sure that there was nothing fundamentally wrong with them other than they were probably more expensive than the Sargent Fletcher. As a result, they would have been used until they were no longer economical to repair.
Note: For a possible third version of the 370-gallon tank, see http://tailspintopics.blogspot.com/2014/02/f4hf-4-370-gallon-external-tank-redux.html