Hohner is a harmonica brand that goes back 150 years, though I don't know how long they've been producing keychain-sized harmonicas like the Pee Wee (which I should add was small enough to be the first musical instrument to go into space), but it's definitely been around long enough for mouth harp fans to learn how to get the most of its little eight note range. Here are a couple videos - one from some 70s TV show (that I really want to see now) that features the Harmonitones (a band comprised entirely of guys described as "my one weird uncle") doing a version of "the Can Can" with the teeny tiny harmonica taking the lead.
The second video is much more modern - a slightly uncomfortable closeup of a man's lips whoofing into his webcam with a mini harmonica. He offers online lessons so that you too can play Star Wars, The Cock O' The North (I don't make this stuff up...), and Old Suzanna on the tiny harmonica that you will be keeping in your pocket at all times from now on.
Help Out Retro Thing When You Pick Up A "Little Lady" Harmonica on Amazon