Ooooooooooooooooooh. Vertical. I love my color copy, and it now costs more to purchase than when I bought mine. I'm resting my left elbow on it right now and will groove on those fine color illustrations after replying to this post. It's such a cool book I may ask the wife to throw it in my coffin when I croak.
I also have a couple copies of the original treatise, one with colored-in black and white illustrations. Well-thumbed copy is still one of my favorites, along with a copy of Montgomery's "Single Rope Techniques" (I'm looking at TWO copies of that in my "working" library beside the computer.) that is actually separating at the spine, it's so used.
I guess if you had print editions you could look inside the frontispiece for the editions information. Let me check mine...