ORIGINAL: ArmChairGeneral
There were a few variants, the British made a M4 crocodile ( based on the Churchill Croc ). The M4 with the flamethrower in the turret did exist, it was no Hollywood error, here are pics of a real one:
http://www.battletanks.com/m4_w_flamethrower.htm
Paul
If I'm reading & understanding GG's post correctly Paul, I think you might have missed the point in what GG was referring to with some of the tanks in the movie possibly just being Hollywood magic.
GG recognized the M4 with the double barrel with turret flamethrower setup as real (almost a side-by-side setup called the POA-CWS-H5), as well as noted other tanks which had flamethrowers mounted in the typical machine gun location in the hull & those that shoot the flames directly from the main barrel. However, I believe GG was probably alluding that there were tanks in the movie which I guess had tubes actually mounted on the top of & maybe even connected to the main barrel, which were shooting the flames.
What GG is looking for then would be some real pictures of a tank that has an over / under setup for the main barrel / flamethrower, with the flamethrower sitting on the top or over the main barrel.
I can't remember from when I had watched the movie long ago, & I couldn't find screen shots of the tanks in question in that movie for examples of the exact FT tanks GG might have been referring to, but I can say that the M4 with the dual setup can easily look like an over / under setup depending on what angle you see the tank, & if you don't get to examine it very closely or for very long, like in this image below ...
~ Craig ~