"Maybe it even belonged to Butch or Sundance themselves"
GM - come on now - let me correct your supposition:
Butch was known for carrying a sawed-off shotgun at times. As I heard from his uncle (Bubba), it was a special model made by Herman S. Inthewal (Senior).
Few folks remember, but Herman's middle name was "Shorty". He designed a shotgun that could be easily carried on a very short mule, without dragging on the ground. Short mules were very cheap back then, and maybe all that was available in Bolivia.
Betcha didn't know that!