I will entirely agree that the strapping on my sheath is designed for left side carry and right side cross draw. Maybe it may actually have been used with the knife in the past as the knife is narrow enough to be inserted, but, as noted, the mouth is quite wide compared with this blade.
Below is an image from
Lew's estate sale of a knife of unknown origin with the cutting edge on the slightly concave long edge that would likely better fill this sheath and in the right direction for left carry, right draw.
My knife is unusual and good observations have been made above. I guess the question is whether it is something left over from Spanish colonial times or totally unrelated.