Don Wilson's search for a 7 mm. hex key took me back to a night in the early 60s. Rice U, Houston, we raced some, and decent bikes were coming to campus (but don't ask about decent students. :-) ). I remember the night we rassled with a Frejus or Legnano which had a 7 mm. stem. There was nothing we could find that night that would loosen it. I finally used a 6 or some near-equivalent SAE, wedging it with a screwdriver blade on one side, and we got it freed up.
But, my sense is that 7 mm was a pretty rare size even back then, or am I wrong? Does anyone know the purported advantages of the 7 over a 6? Yeah, the bolt is a bit lighter because of the bigger hex, but the wrench is heavier. Or was it just to be different?
Clearly, this is not a critical question in the evolution of the bike, but I bet there are some good yarns out there...