The picture of a moving iron headphone looks lifted from either Wikipedia or a collector's website (there's a few guys who restore these things and I'd swear I've seen that picture before). A quick check confirms it was lifted straight from Wikipedia, and is a Brandes set.
Koss himself didn't invent the dynamic headphone nor the stereo headphone, in collaboration with engineers the company basically did for headphones what Ford did for cars (made them affordable and not suck and then marketed them; I'm not trying to say John Koss and Henry Ford are in any way similar people). I'm also unclear on who really invented the first 'stat, the ESP/6 is the first offering from Koss, but not the first electrostatic headphone ever (the STAX SR-1 is nearly ten years older). I also think the Koss ESP/9 or ESP/10 should be up there, as they represent substantial refinements over the ESP/6 - and if we're going to talk about Koss, we have to talk about the Pro4 too.
Koss actually has it's own version of the history of the headphone, but it appears to be down right now. They've added some neat videos though:
http://www.koss.com/community/2011/November/A%20History%20of%20Innovation
Beyerdynamic also has a timeline:
http://north-america.beyerdynamic.com/company/once-today.html (it's also my understanding that all Beyerdynamic headphones come with a little book that goes through most of this, I know the T70 do).
You'd also really need to mention Sony and their influence beyond a simple blurb about the Walkman - it changed a lot more than most people today want to believe it did. It may not be commercially relevant in 2012, but it really did create a new "thing" that everyone else has been following along (this includes the iPod). I would also contend that if we're talking about hi-fi/audiophile stuff, there are a few Sony cans worth mentioning, like the MDR-R10 (which did a lot of things "first" as far as I know; being a woody among them). Sennheiser also needs to be mentioned, as they introduced us to open-back headphones (!!!). You could also talk about some of the weirder products from AKG and Koss over the years (these two more than anyone else, are really obsessed with stuffing more than two drivers into a single pair of headphones). Finally, you might also include Grado, the HP1000 (the Joe Grado stuff) are significant as being among the first "hi-fi" headphones put onto the market in the 1980s (if memory serves; this is along the R10 of course).
Depending on how much you really wanted to get into, you could talk about other technologies too (orthos, electrets, that kind of thing) and the influence of Yamaha, Toshiba, Fostex, and some of the more obscure products from Audio-Technica, Sony, and Beyerdynamic. You could also do in-ears some better justice (I'm not really versed in this, so feel free to fill in where I'm wrong) - the transition from dynamic to BA, and the influence of Etymotic and then UE, Shure, and so on.
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actually, I bet some real head-fi experts could expand and improve on that timeline: anyone? bueller?