Like you said one time design, it’s a term I heard and used a lot when I built doors for yachts. Rich people apparently want the one and only of anything. We built a “one off” (short for one offering I believe) wine cellar door for Francis Ford Copula and that was first time I heard the saying. Anyway the entire staircase on the front of his home lifted up to stairs going into the cellar below. Later we seen pictures with a fog machine and led lights for presentation. Looked like a haunted house.