[Leica] How did 50mm become the "normal" lens for 35mm cameras?

[abridge683 at fastmail.com (Adam Bridge)]

Let me back up a moment.

I was taught that a ?normal? lens is one whose focal length matches the 
diagonal of the film/sensor frame.

For 35mm film as shot by, say, a Leica M6, that turns out to be 43mm - 
midway between a wide 35mm lens and a 50mm lens which I have sometimes seen 
called a ?short telephoto? perhaps even by B&H.

Now as I remember from my cinema days 35mm movies are shot at the same 
aspect ratio as still but have different dimensions since they are shot with 
the long edge running across the film instead of the short edge as is the 
case for full frame 35mm film photography (except, I?m guessing, half-frame 
35mm but now I?m totally out of school having never seen one of these 
cameras.

That?s where I?m coming from. And wondering why we don?t see more 
40-something focal length lenses. I know that there 48?s out there? Nikon 
maybe from the old days? Mark Rabiner, you talked about this at some point, 
I think.

Thanks

Adam