I need biographical information on this German violin maker from the
early 20th Century. I know he was working around 1920, but have been
unable to locate any details of his life or work.

Metzner seems to have been associated with Markneukirchen 
in Vogtland in Germany. "The area developed an internationally
recognized tradition in making musical instruments" See:

http://www.music-centre.com/messe/gesch.htm

You might get help with this question by contacting someone at the
STUDIA INSTRUMENTORUM MUSICAE in Markneukirchen, which is involved in
both research and training. Be warned - I found their website very
slow, with parts still 'under construction'.

http://www.studia-instrumentorum.de/merz.htm

Good luck

Thanks! Maybe I can turn up something there.