That's a Civil War cavalry holster the guy is wearing. You'd wear it like that because you'd draw and fire the revolver with the left hand while the right hand held the reins/saber. You could also draw with the right hand using what is called a "twist draw." Wild Bill Hickok, who served in the Army during the Civil War, carried his Colt Navy revolvers in just such a fashion as a gunfighter.
Look at me learning stuff on my blog!Thanks Bob, that's neat to know :)
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