Well, I had to chime in to say that I agree wholeheartedly! It is without a doubt my favorite song from the American sessions, and that is of course saying something.
I am pretty sure that you are hearing a keyboard in that song.
To my knowledge, only one take of that song has ever been released (on some bootleg or another there is supposedly an alternate, but I believe it is the undubbed master sped up just a tad). That is likely all that exists on tape, but surely there were other attempts? I'm really not a sessions expert in the least though, so someone please correct me if I'm wrong or shed further light on this.