Michael,  When you mention Mixture to an organist, we think of a stop
comprised of two or more ranks of pipes, one of which is usually a
perfect 5th and/or major 3rd.  I don't think this is what fairground
organ builders had in mind.

I have a 52-key Gebr. Bruder Model 107 fair organ.  Looking quickly at
the pipework, I see nothing resembling this.  There are two so-called
Mixtures on this instrument, one soft and one loud.  (Those are
relative terms here as they are both loud.)  Here is a nice link
I got from Markus Fuchs of Orgelbau Fuchs GmbH in Switzerland, which
describes the pipework very well:  http://www.fwbt.ch/SCHW/ORFE.html 

As you will see in the description, the Mixtures are comprised of
a 4' Flute plus a 2' Principal, or otherwise a Principal at both 4'
and 2' pitch -- a perfect octave with no 5ths or 3rds.

Jim Quashnock

 [ A Gebr. Bruder Modell Nr. 107, restored by Orgelbau Fuchs,
 [ plays a potpourri of 1949 hit tunes arranged by Gustav Bruder
 [ at https://www.youtube.com/watch?v=Z06aXkCsww0  -- Robbie