There is indeed a harmonica in there, doing little fills and doubling the lead guitar part. Unfortunately, it's buried in reverb and placed low in the mix so I don't see how you would be able to pick it out with any certainty from the rest of the instruments. The midrange boost is probably just that... some extra EQ added to make it stand out a little more. And drastically compressing the harmonica's dynamic range is certainly not an unheard-of recording practice! That's what gives it that smooth, guitar-like sound. I guess Dave just didn't want to bother with it for live performances, or add a credit for "harmonica - Dave Gilmour" in his liner notes.

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what do you teach your children about me?
what do you teach your little children about me?
pimp, thug, bling drug lord of the undergrounded kings
how can you be so sure i won't call down the rain?
what do you teach your little children about me?
you point your gun, wait, hide and run. i see it plain