Looked it up today and I think I can help you. Y is equal to the diameter of the circle of the largest end piece. Y1 is equal to the vertical distance from mid circle to bottom of the largest end piece. Make the mold a good bit higher than what you will cut your shell pieces so there will be no undercut lip of the end points. This applies to both x and y figures.
X is equal to the diameter of the circle of the smallest end piece.
While I have seen harps with the smallest part of the soundboard up to 5 inches wide, they seem to be very difficult to play due to hand sizes. X1 is equal to the vertical distance from mid circle to bottom of the smallest end piece. On the side view of the plans, I think X should be labeled X1 and Y should be labeled Y1. Although, if you look at the two left drawings you can see where the differences are. You can make one large size to fit your largest harp shell and then cut your material shorter for smaller sized backs using the same relative curve.