The bellows drywasher was developed in northern Mexico and the Southwestern
United States by Mexican prospecters.  It was in common use by the late 19th
century and often refered to as a "Mexican Rocker." This machine was in common
use through the 1930s and is still used by many "weekend" prospectors,
including myself.  They are remarkably efficient.  If you need more detailed
references let me know.
 
 
Stephen Van Wormer
Chula Vista, California