1. Each watermark is uniquely defined as horizontal or vertical because the roll of paper was fed in such a way that the watermark on the roll could not be flipped 90 degrees from the perspective of someone looking at the stamp.
2. The watermark dimensions vary within the same watermark grill. The dimensions that are sometimes quoted for the RA, or any of the relative distance between each RA and its respective rays are of little use.
3. The wavy or straight characteristics of the rays are part of a continuum. While the 2C and 2D watermark have straight rays, the 1E2 and 1E4 also have almost completely straight rays.
Because the large format stamps longer along the horizontal (30c, 40c, 2p) were printed perpendicular to all other stamps, the meaning gets flipped. For example, the 1E2 is a vertical watermark on the 25c, but horizontal on the 30c. If we decided a priori that all watermarks are defined in reference to their positions on all stamps with the exception of the 30c, 40c, and 2p, then we can state that the meaning is flipped for the latter. I am imagining a description as follows:
"We adopt the convention that the watermark is horizontal or vertical in reference to all stamps with the exception of the 30c, 40c, 2p because the latter are printed perpendicular to the direction of all other values of the series. A watermark that is vertical will be horizontal on the 30c, 40c, 2p."
The Bardi conventions are confusing because he does not define his scheme to be unique. That means that when he states that a watermark is horizontal on the 30c, we need to flip this definition and realize that this is a vertical watermark on all stamps other than the 30c, 40c, and 2p.
Here are two illustrative examples of the 1E2, a vertical-only watermark (horizontal on the 30c):
