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I’m not convinced that Marozzo’s guardia d’intrare is with the true edge turned downward.

Ignore the sword in the picture, but look at the hand. The angle is more diagonal than purely downward. http://wiktenauer.com/images/3/3d/Marozzo_20.png

In this edition of opera nova, if again considering the hand only, the true edge is facing inward. http://wiktenauer.com/images/3/3a/Marozzo_50.png

Marozzo’s spada da due mane. The true edge is upward.

I think, what makes guardia d’intrare, is to “enter his line” by turning the body inward, and by passing or moving your left foot (perhaps right foot is also ok) to your inside. Where the sword is is less important. It’s just at where the context requires it to be.