The MASH helicopters had a fuel tank that extended across the frame behind the mast.
As an aside. I've seen this opening scene a thousand times and always wondered if this scene of two machines in formation, flying down the valley was genuine Korea footage?
Certainly the very first and last scene featuring the shows cast obviously isn't, but this pic shows either a very convincing dummy or a human being in the litters. Either those people are injured or
really really calm actors.
As a further aside. For me the best part of the show was seeing Kathy Denny Fradella every night. Who? She's the lead brunette nurse running full tilt to the helipad. I think it was at that point, I knew I wanted to either be a pilot - or get shot.
Edit: This scene is from the movie intro, but I believe it's part of the same footage used for the TV show.