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ValveTime.co.uk | Valve News, Forums, Steam
I tried searching this through the forums and I came up with nothing...
So, the combine_super_soldier.mdl thumbnail included in the SDK...
I've seen the concept art for it and read Valve's excuse for not including it ingame [alien characters in alien enviroments throw off the player's immersion]...
But still. It was a working model at some point.